util/mock/mocks: add package comment.
util/mock/mocks: add package comment.

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