[2027] add comment about doing ACL first

src/lib/python/isc/ddns/session.py

@@ -242,6 +242,9 @@ class UpdateSession:
         '''
         try:
             self._get_update_zone()
+            # Contrary to what RFC2136 specifies, we do ACL checks before
+            # prerequisites. Following the spec, information could leak,
+            # and we decided not to do so (as do other implementations)
             self.__check_update_acl(self.__zname, self.__zclass)
             self._create_diff()
             prereq_result = self.__check_prerequisites()