Comments added

sql/item.h

@@ -997,6 +997,8 @@ class Item_null_helper :public Item_ref_null_helper
   The following class is used to optimize comparing of date and bigint columns
   We need to save the original item, to be able to set the field to the
   original value in 'opt_range'.
+  An instance of Item_int_with_ref may refer to a signed or an unsigned
+  integer.
 */
 
 class Item_int_with_ref :public Item_int