[SPARK-21335][DOC] doc changes for disallowed un-aliased subquery use case
[SPARK-21335][DOC] doc changes for disallowed un-aliased subquery use case

## What changes were proposed in this pull request?
Document a change for un-aliased subquery use case, to address the last question in PR #18559:
https://github.com/apache/spark/pull/18559#issuecomment-316884858

(Please fill in changes proposed in this fix)

## How was this patch tested?
 it does not affect tests.

Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Yuexin Zhang <zach.yx.zhang@gmail.com>

Closes #21647 from cnZach/doc_change_for_SPARK-20690_SPARK-21335.

wzhfy/spark,gengliangwang/spark,WindCanDie/spark,dongjoon-hyun/spark,caneGuy/spark,holdenk/spark,wangmiao1981/spark,gengliangwang/spark,debugger87/spark,mahak/spark,wzhfy/spark,hvanhovell/spark,actuaryzhang/spark,highfei2011/spark,jiangxb1987/spark,rekhajoshm/spark,yanboliang/spark,JoshRosen/spark,chuckchen/spark,kevinyu98/spark,pgandhi999/spark,sahilTakiar/spark,srowen/spark,jkbradley/spark,sahilTakiar/spark,Aegeaner/spark,dongjoon-hyun/spark,bdrillard/spark,vinodkc/spark,milliman/spark,shuangshuangwang/spark,ConeyLiu/spark,cloud-fan/spark,zero323/spark,JoshRosen/spark,zuotingbing/spark,milliman/spark,jiangxb1987/spark,ahnqirage/spark,wzhfy/spark,wzhfy/spark,shaneknapp/spark,darionyaphet/spark,goldmedal/spark,ConeyLiu/spark,jkbradley/spark,tejasapatil/spark,zuotingbing/spark,ueshin/apache-spark,ptkool/spark,ptkool/spark,kevinyu98/spark,gengliangwang/spark,maropu/spark,lvdongr/spark,sahilTakiar/spark,shaneknapp/spark,jiangxb1987/spark,BryanCutler/spark,rezasafi/spark,facaiy/spark,chuckchen/spark,techaddict/spark,ConeyLiu/spark,kiszk/spark,shaneknapp/spark,milliman/spark,bdrillard/spark,michalsenkyr/spark,facaiy/spark,aosagie/spark,WindCanDie/spark,jiangxb1987/spark,wangmiao1981/spark,bravo-zhang/spark,tengpeng/spark,sahilTakiar/spark,shuangshuangwang/spark,wangyum/spark,debugger87/spark,eyalfa/spark,milliman/spark,LantaoJin/spark,srowen/spark,maropu/spark,eyalfa/spark,kiszk/spark,srowen/spark,michalsenkyr/spark,icexelloss/spark,ueshin/apache-spark,srowen/spark,hvanhovell/spark,Aegeaner/spark,rikima/spark,tejasapatil/spark,bravo-zhang/spark,chuckchen/spark,rezasafi/spark,zzcclp/spark,debugger87/spark,dongjoon-hyun/spark,zhouyejoe/spark,pgandhi999/spark,ConeyLiu/spark,apache/spark,Aegeaner/spark,rekhajoshm/spark,Aegeaner/spark,facaiy/spark,taroplus/spark,kevinyu98/spark,tengpeng/spark,cloud-fan/spark,holdenk/spark,hhbyyh/spark,HyukjinKwon/spark,guoxiaolongzte/spark,shaneknapp/spark,rednaxelafx/apache-spark,LantaoJin/spark,holdenk/spark,holdenk/spark,eyalfa/spark,gengliangwang/spark,guoxiaolongzte/spark,maropu/spark,ahnqirage/spark,zzcclp/spark,michalsenkyr/spark,mdespriee/spark,kevinyu98/spark,rekhajoshm/spark,ahnqirage/spark,rikima/spark,vinodkc/spark,ahnqirage/spark,rednaxelafx/apache-spark,wangmiao1981/spark,ahnqirage/spark,taroplus/spark,guoxiaolongzte/spark,JoshRosen/spark,jkbradley/spark,dbtsai/spark,actuaryzhang/spark,aosagie/spark,zero323/spark,tejasapatil/spark,yanboliang/spark,skonto/spark,bdrillard/spark,techaddict/spark,WindCanDie/spark,WeichenXu123/spark,BryanCutler/spark,gengliangwang/spark,mahak/spark,spark-test/spark,wzhfy/spark,holdenk/spark,ptkool/spark,darionyaphet/spark,goldmedal/spark,WindCanDie/spark,JoshRosen/spark,mahak/spark,eyalfa/spark,dbtsai/spark,jkbradley/spark,tengpeng/spark,witgo/spark,mdespriee/spark,vinodkc/spark,lvdongr/spark,spark-test/spark,actuaryzhang/spark,holdenk/spark,skonto/spark,wangyum/spark,yanboliang/spark,cloud-fan/spark,chuckchen/spark,taroplus/spark,eyalfa/spark,ueshin/apache-spark,hhbyyh/spark,mahak/spark,rezasafi/spark,maropu/spark,zzcclp/spark,skonto/spark,bdrillard/spark,rekhajoshm/spark,nchammas/spark,milliman/spark,dongjoon-hyun/spark,spark-test/spark,rikima/spark,zhouyejoe/spark,wangmiao1981/spark,LantaoJin/spark,ConeyLiu/spark,highfei2011/spark,actuaryzhang/spark,witgo/spark,yanboliang/spark,LantaoJin/spark,skonto/spark,BryanCutler/spark,HyukjinKwon/spark,rikima/spark,srowen/spark,JoshRosen/spark,apache/spark,zhouyejoe/spark,wzhfy/spark,xuanyuanking/spark,ptkool/spark,kiszk/spark,apache/spark,skonto/spark,Aegeaner/spark,BryanCutler/spark,Aegeaner/spark,hvanhovell/spark,zzcclp/spark,shaneknapp/spark,zhouyejoe/spark,facaiy/spark,zzcclp/spark,taroplus/spark,kevinyu98/spark,nchammas/spark,nchammas/spark,rekhajoshm/spark,michalsenkyr/spark,caneGuy/spark,highfei2011/spark,ahnqirage/spark,guoxiaolongzte/spark,HyukjinKwon/spark,dongjoon-hyun/spark,caneGuy/spark,goldmedal/spark,xuanyuanking/spark,pgandhi999/spark,goldmedal/spark,zero323/spark,dbtsai/spark,kiszk/spark,kevinyu98/spark,yanboliang/spark,WeichenXu123/spark,facaiy/spark,apache/spark,eyalfa/spark,zero323/spark,srowen/spark,darionyaphet/spark,dbtsai/spark,techaddict/spark,lvdongr/spark,icexelloss/spark,guoxiaolongzte/spark,lvdongr/spark,hhbyyh/spark,wangyum/spark,dbtsai/spark,pgandhi999/spark,mdespriee/spark,gengliangwang/spark,bravo-zhang/spark,aosagie/spark,WeichenXu123/spark,WindCanDie/spark,spark-test/spark,mdespriee/spark,nchammas/spark,ConeyLiu/spark,rednaxelafx/apache-spark,spark-test/spark,eyalfa/spark,debugger87/spark,chuckchen/spark,mahak/spark,WeichenXu123/spark,chuckchen/spark,rezasafi/spark,rednaxelafx/apache-spark,ptkool/spark,cloud-fan/spark,matthewfranglen/spark,srowen/spark,mdespriee/spark,debugger87/spark,aosagie/spark,michalsenkyr/spark,BryanCutler/spark,WindCanDie/spark,maropu/spark,tengpeng/spark,HyukjinKwon/spark,hvanhovell/spark,hvanhovell/spark,tengpeng/spark,shaneknapp/spark,bdrillard/spark,witgo/spark,darionyaphet/spark,WeichenXu123/spark,hhbyyh/spark,tengpeng/spark,taroplus/s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