Modify auc doc. Add output variable description, previously was the scalar type, now changed to the tuple type.test=develop !18767
Created by: JesseyXujin
output variable previously was scalar(auc), now change to tuple(auc,batch_auc,[batch_stat_pos,batch_stat_neg,stat_pos,stat_neg]) issue: http://newicafe.baidu.com:80/issue/DLTP-1572/show?from=page