[SPARK-18812][MLLIB] explain "Spark ML"
[SPARK-18812][MLLIB] explain "Spark ML"

## What changes were proposed in this pull request?

There has been some confusion around "Spark ML" vs. "MLlib". This PR adds some FAQ-like entries to the MLlib user guide to explain "Spark ML" and reduce the confusion.

I check the [Spark FAQ page](http://spark.apache.org/faq.html), which seems too high-level for the content here. So I added it to the MLlib user guide instead.

cc: mateiz

Author: Xiangrui Meng <meng@databricks.com>

Closes #16241 from mengxr/SPARK-18812.

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