[SPARK-14882][DOCS] Clarify that Spark can be cross-built for other Scala versions
[SPARK-14882][DOCS] Clarify that Spark can be cross-built for other Scala versions

## What changes were proposed in this pull request?

Add simple clarification that Spark can be cross-built for other Scala versions.

## How was this patch tested?

Automated doc build

Author: Sean Owen <sowen@cloudera.com>

Closes #12757 from srowen/SPARK-14882.

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