rewrite comments

planner/core/logical_plan_builder.go

@@ -310,9 +310,9 @@ func (p *LogicalJoin) extractOnCondition(conditions []expression.Expression, der
 	return
 }
 
-// This function will return the table alias of a given logical plan.
-// If there're multiple tables, it will return nil.
-// This function will be further developed in the future.
+// The table alias is only used to check if the logicalPlan match some
+// optimizer hints. It will return nil when there are multiple table alias,
+// because hints are not expected to take effect in this case.
 func extractTableAlias(p LogicalPlan) *model.CIStr {
 	if p.Schema().Len() > 0 && p.Schema().Columns[0].TblName.L != "" {
 		tblName := p.Schema().Columns[0].TblName.L