[SPARK-8457] [ML] NGram Documentation
[SPARK-8457] [ML] NGram Documentation

Add documentation for NGram feature transformer.

Author: Feynman Liang <fliang@databricks.com>

Closes #7244 from feynmanliang/SPARK-8457 and squashes the following commits:

5aface9 [Feynman Liang] Pretty print Scala output and add API doc to each codetab
60d5ac0 [Feynman Liang] Inline API doc and fix indentation
736ccbc [Feynman Liang] NGram feature transformer documentation

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