[SPARK-13748][PYSPARK][DOC] Add the description for explictly setting None for a named argument for a Row
[SPARK-13748][PYSPARK][DOC] Add the description for explictly setting None for a named argument for a Row

## What changes were proposed in this pull request?

It seems allowed to not set a key and value for a dict to represent the value is `None` or missing as below:

``` python
spark.createDataFrame([{"x": 1}, {"y": 2}]).show()
```

```
+----+----+
|   x|   y|
+----+----+
|   1|null|
|null|   2|
+----+----+
```

However,  it seems it is not for `Row` as below:

``` python
spark.createDataFrame([Row(x=1), Row(y=2)]).show()
```

``` scala
16/06/19 16:25:56 ERROR Executor: Exception in task 6.0 in stage 66.0 (TID 316)
java.lang.IllegalStateException: Input row doesn't have expected number of values required by the schema. 2 fields are required while 1 values are provided.
    at org.apache.spark.sql.execution.python.EvaluatePython$.fromJava(EvaluatePython.scala:147)
    at org.apache.spark.sql.SparkSession$$anonfun$7.apply(SparkSession.scala:656)
    at org.apache.spark.sql.SparkSession$$anonfun$7.apply(SparkSession.scala:656)
    at scala.collection.Iterator$$anon$11.next(Iterator.scala:409)
    at scala.collection.Iterator$$anon$11.next(Iterator.scala:409)
    at org.apache.spark.sql.execution.SparkPlan$$anonfun$4.apply(SparkPlan.scala:247)
    at org.apache.spark.sql.execution.SparkPlan$$anonfun$4.apply(SparkPlan.scala:240)
    at org.apache.spark.rdd.RDD$$anonfun$mapPartitionsInternal$1$$anonfun$apply$24.apply(RDD.scala:780)
```

The behaviour seems right but it seems it might confuse users just like this JIRA was reported.

This PR adds the explanation for `Row` class.
## How was this patch tested?

N/A

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #13771 from HyukjinKwon/SPARK-13748.

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