Fix comment

Author: wjhuang2016

expression/expression.go

@@ -79,7 +79,12 @@ type Expression interface {
 	// IsCorrelated checks if this expression has correlated key.
 	IsCorrelated() bool
 
-	// ConstItem checks if this expression refers to a table.
+	// ConstItem checks if this expression is constant item, regardless of query evaluation state.
+	// An expression is constant item if it:
+	// refers no tables.
+	// refers no subqueries that refers any tables.
+	// refers no non-deterministic functions.
+	// refers no statement parameters.
 	ConstItem() bool
 
 	// Decorrelate try to decorrelate the expression by schema.