[SPARK-18793][SPARK-18794][R] add spark.randomForest/spark.gbt to vignettes
[SPARK-18793][SPARK-18794][R] add spark.randomForest/spark.gbt to vignettes

## What changes were proposed in this pull request?

Mention `spark.randomForest` and `spark.gbt` in vignettes. Keep the content minimal since users can type `?spark.randomForest` to see the full doc.

cc: jkbradley

Author: Xiangrui Meng <meng@databricks.com>

Closes #16264 from mengxr/SPARK-18793.

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