[SPARK-12570][ML][DOC] DecisionTreeRegressor: provide variance of prediction: user guide update
[SPARK-12570][ML][DOC] DecisionTreeRegressor: provide variance of prediction: user guide update

Update user guide doc for ```DecisionTreeRegressor``` providing variance of prediction.

cc jkbradley

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #10594 from yanboliang/spark-12570.

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