[SPARK-1701] Clarify slice vs partition in the programming guide
[SPARK-1701] Clarify slice vs partition in the programming guide

This is a partial solution to SPARK-1701, only addressing the
documentation confusion.

Additional work can be to actually change the numSlices parameter name
across languages, with care required for scala & python to maintain
backward compatibility for named parameters.

Author: Matthew Farrellee <matt@redhat.com>

Closes #2305 from mattf/SPARK-1701 and squashes the following commits:

c0af05d [Matthew Farrellee] Further tweak
06f80fc [Matthew Farrellee] Wording tweak from Josh Rosen's review
7b045e0 [Matthew Farrellee] [SPARK-1701] Clarify slice vs partition in the programming guide

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