logging: clarified Filter documentation.

Doc/library/logging.rst

@@ -2708,6 +2708,9 @@ etc.) This means that events which have been generated by descendant loggers
 will not be filtered by a logger's filter setting, unless the filter has also
 been applied to those descendant loggers.
 
+You don't actually need to subclass ``Filter``: you can pass any instance
+which has a ``filter`` method with the same semantics.
+
 Other uses for filters
 ^^^^^^^^^^^^^^^^^^^^^^