ast: add comments on ResultSetNode interface.
ast: add comments on ResultSetNode interface.

qiuyesuifeng/tidb,silenceper/tidb,qhsong/tidb,8cbx/tidb,CodeRushing/tidb,wusphinx/tidb,yangwenmai/tidb,anywhy/tidb,EvilMcJerkface/tidb,louishust/tidb,kyle-wang/tidb,wusphinx/tidb,yangwenmai/tidb,myrice/tidb,gaocegege/tidb,ngaut/tidb,Nify/tidb,zimulala/tidb,ueizhou/tidb,c4pt0r/tidb,ngaut/tidb,kyle-wang/tidb,mccxj/tidb,c4pt0r/tidb,ngaut/tidb,ming-relax/tidb,andelf/tidb,zs634134578/tidb,kyle-wang/tidb,andelf/tidb,pingcap/tidb,simon-xia/tidb,cosmtrek/tidb,liyu4/tidb,JinheLin/tidb,coocood/tidb,netroby/tidb,pingcap/tidb,Cofyc/tidb,ming-relax/tidb,c4pt0r/tidb,ngaut/tidb,Cofyc/tidb,qhsong/tidb,c4pt0r/tidb,silentred/tidb,qiuyesuifeng/tidb,tiancaiamao/tidb,pingcap/tidb,anywhy/tidb,kyle-wang/tidb,kyle-wang/tidb,XuHuaiyu/tidb,astaxie/tidb,ngaut/tidb,Nify/tidb,netroby/tidb,spongedu/tidb,cosmtrek/tidb,kyle-wang/tidb,pingcap/tidb,gaocegege/tidb,zs634134578/tidb,c4pt0r/tidb,spongedu/tidb,astaxie/tidb,myrice/tidb,8cbx/tidb,pingcap/tidb,liyu4/tidb,JinheLin/tidb,EvilMcJerkface/tidb,ueizhou/tidb,yuanwhy/tidb,XuHuaiyu/tidb,coocood/tidb,simon-xia/tidb,mccxj/tidb,louishust/tidb,zimulala/tidb,CodeRushing/tidb,silenceper/tidb,silentred/tidb,c4pt0r/tidb,pingcap/tidb,ngaut/tidb,tiancaiamao/tidb,yuanwhy/tidb