parser: add comment for exported variables.
parser: add comment for exported variables.

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