[MINOR][DOC] improve the doc for "spark.memory.offHeap.size"
[MINOR][DOC] improve the doc for "spark.memory.offHeap.size"

The description of "spark.memory.offHeap.size" in the current document does not clearly state that memory is counted with bytes....

This PR contains a small fix for this tiny issue

document fix

Author: CodingCat <zhunansjtu@gmail.com>

Closes #11561 from CodingCat/master.

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