comment to user-level lock added

sql/sql_class.h

@@ -505,6 +505,11 @@ class THD :public ilink
   // TODO: document the variables below
   MYSQL_LOCK	*lock;				/* Current locks */
   MYSQL_LOCK	*locked_tables;			/* Tables locked with LOCK */
+  /*
+    One thread can hold up to one named user-level lock. This variable
+    points to a lock object if the lock is present. See item_func.cc and
+    chapter 'Miscellaneous functions', for functions GET_LOCK, RELEASE_LOCK. 
+  */
   ULL		*ull;
   PREP_STMT	*last_prepared_stmt;
 #ifndef DBUG_OFF