more comment update

git-svn-id: svn://bind10.isc.org/svn/bind10/branches/trac275@2445 e5f2f494-b856-4b98-b285-d166d9295462

src/lib/config/ccsession.h

@@ -123,6 +123,8 @@ public:
      * data are exchanged.
      * @param config_handler A callback function pointer to be called when
      * configuration of the local module needs to be updated.
+     * This must refer to a valid object of a concrete derived class of
+     * AbstractSession without establishing the session.
      * @param command_handler A callback function pointer to be called when
      * a control command from a remote agent needs to be performed on the
      * local module.