Complete the list of weakreferencable objects.

Doc/lib/libweakref.tex

@@ -49,6 +49,7 @@ by the \module{weakref} module for the benefit of advanced uses.
 Not all objects can be weakly referenced; those objects which can
 include class instances, functions written in Python (but not in C),
 methods (both bound and unbound), sets, frozensets, file objects,
+generators, type objects, DBcursor objects from the \module{bsddb} module,
 sockets, arrays, deques, and regular expression pattern objects.
 \versionchanged[Added support for files, sockets, arrays, and patterns]{2.4}