[SPARK-2945][YARN][Doc]add doc for spark.executor.instances
[SPARK-2945][YARN][Doc]add doc for spark.executor.instances

https://issues.apache.org/jira/browse/SPARK-2945

spark.executor.instances works. As this JIRA recommended, we should add docs for this common config.

Author: WangTaoTheTonic <wangtao111@huawei.com>

Closes #4350 from WangTaoTheTonic/SPARK-2945 and squashes the following commits:

4c3913a [WangTaoTheTonic] not compatible with dynamic allocation
5fa9c46 [WangTaoTheTonic] add doc for spark.executor.instances

byakuinss/spark,sahilTakiar/spark,WeichenXu123/spark,rednaxelafx/apache-spark,debugger87/spark,metamx/spark,bOOm-X/spark,highfei2011/spark,haowu80s/spark,shuangshuangwang/spark,lxsmnv/spark,zzcclp/spark,actuaryzhang/spark,vinodkc/spark,jlopezmalla/spark,publicRoman/spark,dongjoon-hyun/spark,srowen/spark,andrewor14/spark,chuckchen/spark,srowen/spark,mdespriee/spark,holdenk/spark,kimoonkim/spark,lxsmnv/spark,brad-kaiser/spark,nchammas/spark,skonto/spark,haowu80s/spark,mzl9039/spark,ron8hu/spark,kiszk/spark,Panos-Bletsos/spark-cost-model-optimizer,jiangxb1987/spark,chuckchen/spark,ddna1021/spark,ConeyLiu/spark,jrshust/spark,aray/spark,guoxiaolongzte/spark,SnappyDataInc/spark,akopich/spark,Aegeaner/spark,darionyaphet/spark,aray/spark,pgandhi999/spark,ConeyLiu/spark,JoshRosen/spark,ahnqirage/spark,akopich/spark,caneGuy/spark,chuckchen/spark,nchammas/spark,huang1900/spark,joseph-torres/spark,haowu80s/spark,szhem/spark,liyichao/spark,pronix/spark,kevinyu98/spark,SHASHANKB/spark,ddna1021/spark,UndeadBaneGitHub/spark,rekhajoshm/spark,shubhamchopra/spark,HyukjinKwon/spark,UndeadBaneGitHub/spark,ron8hu/spark,mzl9039/spark,jiangxb1987/spark,gengliangwang/spark,dhruve/spark,jlopezmalla/spark,metamx/spark,jrshust/spark,HyukjinKwon/spark,vinodkc/spark,eyalfa/spark,hvanhovell/spark,apache-spark-on-k8s/spark,nchammas/spark,mahak/spark,lvdongr/spark,WindCanDie/spark,nlalevee/spark,xuanyuanking/spark,nilsgrabbert/spark,caneGuy/spark,JerryLead/spark,ioana-delaney/spark,eyalfa/spark,dongjoon-hyun/spark,pgandhi999/spark,alunarbeach/spark,eyalfa/spark,gioenn/xSpark,adrian-ionescu/apache-spark,rezasafi/spark,spark-test/spark,wzhfy/spark,milliman/spark,saturday-shi/spark,liutang123/spark,wangyum/spark,apache-spark-on-k8s/spark,rednaxelafx/apache-spark,ioana-delaney/spark,hvanhovell/spark,ioana-delaney/spark,aosagie/spark,cin/spark,WindCanDie/spark,patrick-nicholson/spark,saltstar/spark,narahari92/spark,metamx/spark,yanboliang/spark,pgandhi999/spark,ahnqirage/spark,xuanyuanking/spark,hvanhovell/spark,andrewor14/spark,huang1900/spark,wangyum/spark,ConeyLiu/spark,Aegeaner/spark,facaiy/spark,holdenk/spark,SnappyDataInc/spark,skonto/spark,poffuomo/spark,apache/spark,apache/spark,minixalpha/spark,ericvandenbergfb/spark,brad-kaiser/spark,zuotingbing/spark,haowu80s/spark,tengpeng/spark,esi-mineset/spark,taroplus/spark,aokolnychyi/spark,mahak/spark,aokolnychyi/spark,LantaoJin/spark,poffuomo/spark,mahak/spark,stanzhai/spark,matthewfranglen/spark,setjet/spark,techaddict/spark,lxsmnv/spark,janewangfb/spark,guoxiaolongzte/spark,poffuomo/spark,ajaysaini725/spark,dongjoon-hyun/spark,gengliangwang/spark,HyukjinKwon/spark,brad-kaiser/spark,SnappyDataInc/spark,aray/spark,darionyaphet/spark,wzhfy/spark,zuotingbing/spark,ajaysaini725/spark,wzhfy/spark,lvdongr/spark,nlalevee/spark,darionyaphet/spark,liutang123/spark,rekhajoshm/spark,wangmiao1981/spark,akopich/spark,hhbyyh/spark,adrian-ionescu/apache-spark,minixalpha/spark,gengliangwang/spark,dhruve/spark,gioenn/xSpark,andrewor14/iolap,ajaysaini725/spark,goldmedal/spark,shuangshuangwang/spark,setjet/spark,WindCanDie/spark,kevinyu98/spark,pgandhi999/spark,bdrillard/spark,eyalfa/spark,liutang123/spark,dhruve/spark,1haodian/spark,aosagie/spark,lxsmnv/spark,mdespriee/spark,jiangxb1987/spark,srowen/spark,saltstar/spark,HyukjinKwon/spark,SHASHANKB/spark,map222/spark,guoxiaolongzte/spark,icexelloss/spark,xflin/spark,liutang123/spark,liutang123/spark,BryanCutler/spark,cin/spark,shaneknapp/spark,BryanCutler/spark,bOOm-X/spark,shaneknapp/spark,map222/spark,markhamstra/spark,stanzhai/spark,janewangfb/spark,ptkool/spark,aosagie/spark,map222/spark,sahilTakiar/spark,spark-test/spark,actuaryzhang/spark,SnappyDataInc/spark,HyukjinKwon/spark,rednaxelafx/apache-spark,liyichao/spark,joseph-torres/spark,ajaysaini725/spark,ibm-research-ireland/sparkoscope,WeichenXu123/spark,zzcclp/spark,techaddict/spark,spark-test/spark,publicRoman/spark,gengliangwang/spark,rednaxelafx/apache-spark,xflin/spark,wangmiao1981/spark,nchammas/spark,narahari92/spark,jiangxb1987/spark,tejasapatil/spark,cloud-fan/spark,shaneknapp/spark,bdrillard/spark,nilsgrabbert/spark,someorz/spark,kevinyu98/spark,MLnick/spark,hhbyyh/spark,map222/spark,cin/spark,ueshin/apache-spark,markhamstra/spark,zuotingbing/spark,mzl9039/spark,cin/spark,jrshust/spark,jkbradley/spark,andrewor14/iolap,MLnick/spark,actuaryzhang/spark,tejasapatil/spark,szhem/spark,debugger87/spark,JoshRosen/spark,dbtsai/spark,ioana-delaney/spark,xflin/spark,huang1900/spark,jkbradley/spark,zero323/spark,kissmetrics/spark,mike0sv/spark,poffuomo/spark,tengpeng/spark,lxsmnv/spark,apache-spark-on-k8s/spark,1haodian/spark,adrian-ionescu/apache-spark,saltstar/spark,wangyum/spark,stanzhai/spark,akopich/spark,ron8hu/spark,rikima/spark,mzl9039/spark,guoxiaolongzte/spark,cin/spark,kissmetrics/spark,rikima/spark,SnappyDataInc/spark,zero323/spark,zhouyejoe/spark,Panos-Bletsos/spark-cost-model-optimizer,aray/spark,szhem/spark,goldmedal/spark,ericvandenbergfb/spark,apache/spark,JoshRosen/spark,shubhamchopra/spark,icexelloss/spark,saturday-shi/spark,someorz/spark,jianran/spark,jlopezmalla/spark,sachintyagi22/spark,adrian-ionescu/apache-spark,dhruve/spark,WindCanDie/spark,witgo/spark,aokolnychyi/spark,kevinyu98/spark,sachintyagi22/spark,LantaoJin/spark,publicRoman/spark,sahilTakiar/spark,rekhajoshm/spark,sachintyagi22/spark,yanboliang/spark,jkbradley/spark,mdespriee/spark,kiszk/spark,someorz/spark,Aegeaner/spark,LantaoJin/spark,shubhamchopra/spark,dbtsai/spark,holdenk/spark,debugger87/spark,vinodkc/spark,zhouyejoe/spark,stanzhai/spark,witgo/spark,ibm-research-ireland/sparkoscope,rezasafi/spark,zhouyejoe/spark,guoxiaolongzte/spark,ueshin/apache-spark,bravo-zhang/spark,SHASHANKB/spark,metamx/spark,dbtsai/spark,techaddict/spark,aray/spark,bOOm-X/spark,maropu/spark,ioana-delaney/spark,apache-spark-on-k8s/spark,facaiy/spark,srowen/spark,tejasapatil/spark,xflin/spark,eyalfa/spark,techaddict/spark,holdenk/spark,srowen/spark,kimoonkim/spark,1haodian/spark,bOOm-X/spark,sahilTakiar/spark,panfengfeng/spark,witgo/spark,kiszk/spark,michalsenkyr/spark,skonto/spark,nchammas/spark,yanboliang/spark,rikima/spark,mdespriee/spark,bdrillard/spark,darionyaphet/spark,yanboliang/spark,1haodian/spark,ptkool/spark,guoxiaolongzte/spark,big-pegasus/spark,sachintyagi22/spark,liyichao/spark,panfengfeng/spark,aokolnychyi/spark,jianran/spark,someorz/spark,jianran/spark,map222/spark,debugger87/spark,kissmetrics/spark,loneknightpy/spark,1haodian/spark,ericvandenbergfb/spark,lxsmnv/spark,shuangshuangwang/spark,nchammas/spark,wangmiao1981/spark,szhem/spark,spark-test/spark,ron8hu/spark,narahari92/spark,janewangfb/spark,jianran/spark,sureshthalamati/spark,szhem/spark,lxsmnv/spark,Aegeaner/spark,highfei2011/spark,adrian-ionescu/apache-spark,ahnqirage/spark,minixalpha/spark,BryanCutler/spark,shuangshuangwang/spark,big-pegasus/spark,actuaryzhang/spark,shaneknapp/spark,alunarbeach/spark,nilsgrabbert/spark,big-pegasus/spark,xuanyuanking/spark,eyalfa/spark,zuotingbing/spark,sureshthalamati/spark,sachintyagi22/spark,icexelloss/spark,icexelloss/spark,witgo/spark,rednaxelafx/apache-spark,tejasapatil/spark,jiangxb1987/spark,andrewor14/iolap,techaddict/spark,adrian-ionescu/apache-spark,Panos-Bletsos/spark-cost-model-optimizer,dhruve/spark,hhbyyh/spark,SHASHANKB/spark,srowen/spark,bOOm-X/spark,bdrillard/spark,huang1900/spark,jkbradley/spark,saturday-shi/spark,brad-kaiser/spark,actuaryzhang/spark,setjet/spark,nlalevee/spark,debugger87/spark,apache/spark,zzcclp/spark,wzhfy/spark,esi-mineset/spark,highfei2011/spark,huang1900/spark,michalsenkyr/spark,panfengfeng/spark,someorz/spark,markhamstra/spark,markhamstra/spark,sureshthalamati/spark,shubhamchopra/spark,stanzhai/spark,actuaryzhang/spark,zhouyejoe/spark,JerryLead/spark,MLnick/spark,mike0sv/spark,ddna1021/spark,alunarbeach/spark,highfei2011/spark,andrewor14/iolap,kimoonkim/spark,wangyum/spark,panfengfeng/spark,jrshust/spark,wangmiao1981/spark,ueshin/apache-spark,bdrillard/spark,szhem/spark,1haodian/spark,tengpeng/spark,techaddict/spark,milliman/spark,taroplus/spark,pgandhi999/spark,byakuinss/spark,SHASHANKB/spark,mahak/spark,ueshin/apache-spark,rekhajoshm/spark,SnappyDataInc/spark,saturday-shi/spark,jkbradley/spark,WeichenXu123/spark,witgo/spark,holdenk/spark,witgo/spark,mike0sv/spark,milliman/spark,kiszk/spark,debugger87/spark,kevinyu98/spark,dhruve/spark,ahnqirage/spark,ioana-delaney/spark,panfengfeng/spark,jiangxb1987/spark,facaiy/spark,cin/spark,janewangfb/spark,byakuinss/spark,Aegeaner/spark,caneGuy/spark,stanzhai/spark,tengpeng/spark,metamx/spark,zzcclp/spark,poffuomo/spark,skonto/spark,big-pegasus/spark,cloud-fan/spark,apache-spark-on-k8s/spark,ahnqirage/spark,nlalevee/spark,zero323/spark,kimoonkim/spark,saturday-shi/spark,cloud-fan/spark,cloud-fan/spark,caneGuy/spark,zuotingbing/spark,facaiy/spark,mike0sv/spark,MLnick/spark,darionyaphet/spark,Aegeaner/spark,rezasafi/spark,zhouyejoe/spark,skonto/spark,nilsgrabbert/spark,joseph-torres/spark,narahari92/spark,panfengfeng/spark,rezasafi/spark,lvdongr/spark,andrewor14/spark,apache/spark,sachintyagi22/spark,vinodkc/spark,ueshin/apache-spark,MLnick/spark,sahilTakiar/spark,gengliangwang/spark,SHASHANKB/spark,publicRoman/spark,esi-mineset/spark,zuotingbing/spark,JerryLead/spark,techaddict/spark,akopich/spark,yanboliang/spark,facaiy/spark,wangmiao1981/spark,goldmedal/spark,pronix/spark,cloud-fan/spark,cin/spark,michalsenkyr/spark,joseph-torres/spark,ddna1021/spark,liyichao/spark,Panos-Bletsos/spark-cost-model-optimizer,dbtsai/spark,alunarbeach/spark,mdespriee/spark,akopich/spark,alunarbeach/spark,jrshust/spark,1haodian/spark,chuckchen/spark,janewangfb/spark,ericvandenbergfb/spark,ahnqirage/spark,lvdongr/spark,taroplus/spark,JoshRosen/spark,andrewor14/iolap,ConeyLiu/spark,HyukjinKwon/spark,mdespriee/spark,jlopezmalla/spark,zero323/spark,joseph-torres/spark,goldmedal/spark,kiszk/spark,tejasapatil/spark,maropu/spark,patrick-nicholson/spark,shaneknapp/spark,szhem/spark,rednaxelafx/apache-spark,xflin/spark,jiangxb1987/spark,stanzhai/spark,goldmedal/spark,ibm-research-ireland/sparkoscope,yanboliang/spark,ericvandenbergfb/spark,minixalpha/spark,nlalevee/spark,vinodkc/spark,LantaoJin/spark,bdrillard/spark,xflin/spark,kissmetrics/spark,kiszk/spark,icexelloss/spark,saltstar/spark,liutang123/spark,cloud-fan/spark,lvdongr/spark,gengliangwang/spark,narahari92/spark,LantaoJin/spark,apache/spark,maropu/spark,vinodkc/spark,skonto/spark,holdenk/spark,publicRoman/spark,WindCanDie/spark,patrick-nicholson/spark,pronix/spark,liutang123/spark,ioana-delaney/spark,debugger87/spark,bravo-zhang/spark,someorz/spark,poffuomo/spark,pronix/spark,big-pegasus/spark,dbtsai/spark,sachintyagi22/spark,mzl9039/spark,ajaysaini725/spark,aokolnychyi/spark,kimoonkim/spark,maropu/spark,caneGuy/spark,JoshRosen/spark,MLnick/spark,jianran/spark,ptkool/spark,brad-kaiser/spark,andrewor14/iolap,jrshust/spark,loneknightpy/spark,taroplus/spark,chuckchen/spark,sureshthalamati/spark,mike0sv/spark,aosagie/spark,haowu80s/spark,aray/spark,hvanhovell/spark,loneknightpy/spark,setjet/spark,zzcclp/spark,patrick-nicholson/spark,taroplus/spark,mzl9039/spark,shubhamchopra/spark,dongjoon-hyun/spark,tengpeng/spark,mahak/spark,hhbyyh/spark,spark-test/spark,spark-test/spark,mike0sv/spark,brad-kaiser/spark,xuanyuanking/spark,BryanCutler/spark,kevinyu98/spark,zhouyejoe/spark,jrshust/spark,spark-test/spark,michalsenkyr/spark,gioenn/xSpark,minixalpha/spark,loneknightpy/spark,wangmiao1981/spark,facaiy/spark,milliman/spark,facaiy/spark,xflin/spark,bOOm-X/spark,jlopezmalla/spark,hhbyyh/spark,kimoonkim/spark,SnappyDataInc/spark,bdrillard/spark,kissmetrics/spark,holdenk/spark,ddna1021/spark,sureshthalamati/spark,jkbradley/spark,ajaysaini725/spark,bravo-zhang/spark,someorz/spark,narahari92/spark,markhamstra/spark,darionyaphet/spark,aosagie/spark,bOOm-X/spark,haowu80s/spark,aokolnychyi/spark,wangyum/spark,goldmedal/spark,saltstar/spark,apache/spark,kiszk/spark,pronix/spark,jianran/spark,saturday-shi/spark,nchammas/spark,ajaysaini725/spark,dhruve/spark,nlalevee/spark,zuotingbing/spark,ptkool/spark,huang1900/spark,shuangshuangwang/spark,sureshthalamati/spark,darionyaphet/spark,chuckchen/spark,rikima/spark,MLnick/spark,rezasafi/spark,rekhajoshm/spark,jlopezmalla/spark,rednaxelafx/apache-spark,apache-spark-on-k8s/spark,UndeadBaneGitHub/spark,saltstar/spark,nilsgrabbert/spark,gengliangwang/spark,ahnqirage/spark,icexelloss/spark,gioenn/xSpark,ueshin/apache-spark,joseph-torres/spark,pgandhi999/spark,hhbyyh/spark,nlalevee/spark,milliman/spark,brad-kaiser/spark,saturday-shi/spark,UndeadBaneGitHub/spark,michalsenkyr/spark,gioenn/xSpark,milliman/spark,publicRoman/spark,highfei2011/spark,caneGuy/spark,UndeadBaneGitHub/spark,JerryLead/spark,michalsenkyr/spark,shubhamchopra/spark,zero323/spark,tengpeng/spark,ConeyLiu/spark,bravo-zhang/spark,LantaoJin/spark,jianran/spark,rikima/spark,kevinyu98/spark,WeichenXu123/spark,WindCanDie/spark,JerryLead/spark,rekhajoshm/spark,patrick-nicholson/spark,rezasafi/spark,jkbradley/spark,mdespriee/spark,andrewor14/spark,byakuinss/spark,dongjoon-hyun/spark,loneknightpy/spark,lvdongr/spark,rikima/spark,taroplus/spark,ron8hu/spark,UndeadBaneGitHub/spark,goldmedal/spark,chuckchen/spark,markhamstra/spark,xuanyuanking/spark,patrick-nicholson/spark,liyichao/spark,esi-mineset/spark,nilsgrabbert/spark,UndeadBaneGitHub/spark,aokolnychyi/spark,ibm-research-ireland/sparkoscope,janewangfb/spark,hvanhovell/spark,tejasapatil/spark,mahak/spark,huang1900/spark,ConeyLiu/spark,Panos-Bletsos/spark-cost-model-optimizer,gioenn/xSpark,markhamstra/spark,LantaoJin/spark,cloud-fan/spark,milliman/spark,eyalfa/spark,panfengfeng/spark,JoshRosen/spark,JerryLead/spark,highfei2011/spark,xuanyuanking/spark,minixalpha/spark,kimoonkim/spark,pgandhi999/spark,ddna1021/spark,wangyum/spark,ueshin/apache-spark,dbtsai/spark,srowen/spark,shuangshuangwang/spark,andrewor14/spark,byakuinss/spark,dbtsai/spark,ron8hu/spark,highfei2011/spark,skonto/spark,gioenn/xSpark,ericvandenbergfb/spark,pronix/spark,WeichenXu123/spark,mike0sv/spark,esi-mineset/spark,rekhajoshm/spark,HyukjinKwon/spark,mzl9039/spark,ericvandenbergfb/spark,minixalpha/spark,wangyum/spark,map222/spark,big-pegasus/spark,joseph-torres/spark,aray/spark,setjet/spark,janewangfb/spark,zero323/spark,setjet/spark,andrewor14/spark,esi-mineset/spark,Aegeaner/spark,dongjoon-hyun/spark,publicRoman/spark,nilsgrabbert/spark,ibm-research-ireland/sparkoscope,mahak/spark,actuaryzhang/spark,JoshRosen/spark,shaneknapp/spark,dongjoon-hyun/spark,BryanCutler/spark,lvdongr/spark,guoxiaolongzte/spark,ptkool/spark,apache-spark-on-k8s/spark,loneknightpy/spark,ibm-research-ireland/sparkoscope,metamx/spark,adrian-ionescu/apache-spark,WindCanDie/spark,saltstar/spark,zzcclp/spark,WeichenXu123/spark,akopich/spark,sahilTakiar/spark,shubhamchopra/spark,poffuomo/spark,hhbyyh/spark,liyichao/spark,jlopezmalla/spark,BryanCutler/spark,maropu/spark,SHASHANKB/spark,alunarbeach/spark,bravo-zhang/spark,xuanyuanking/spark,vinodkc/spark,wangmiao1981/spark,byakuinss/spark,byakuinss/spark,rikima/spark,loneknightpy/spark,zero323/spark,liyichao/spark,map222/spark,Panos-Bletsos/spark-cost-model-optimizer,caneGuy/spark,ron8hu/spark,ddna1021/spark,maropu/spark,sureshthalamati/spark,taroplus/spark,rezasafi/spark,ptkool/spark,aosagie/spark,JerryLead/spark,wzhfy/spark,icexelloss/spark,yanboliang/spark,esi-mineset/spark,narahari92/spark,alunarbeach/spark,michalsenkyr/spark,patrick-nicholson/spark,ConeyLiu/spark,zhouyejoe/spark,aosagie/spark,wzhfy/spark,hvanhovell/spark,hvanhovell/spark,kissmetrics/spark,big-pegasus/spark,ibm-research-ireland/sparkoscope,andrewor14/spark,Panos-Bletsos/spark-cost-model-optimizer,wzhfy/spark,WeichenXu123/spark,tengpeng/spark,sahilTakiar/spark,setjet/spark,bravo-zhang/spark,tejasapatil/spark,metamx/spark,ptkool/spark,shuangshuangwang/spark,maropu/spark,andrewor14/iolap,witgo/spark,BryanCutler/spark,shaneknapp/spark,bravo-zhang/spark,zzcclp/spark,kissmetrics/spark