Include `versionadded 1.5` directive

docs/ref/class-based-views/mixins-multiple-object.txt

@@ -74,6 +74,8 @@ MultipleObjectMixin
 
     .. attribute:: page_kwarg
 
+        .. versionadded:: 1.5
+
         A string specifying the name to use for the page parameter.
         The view will expect this prameter to be available either as a query
         string parameter (via ``request.GET``) or as a kwarg variable specified