rewrite to be nice to other implementations

Doc/library/weakref.rst

@@ -77,8 +77,8 @@ support weak references but can add support through subclassing::
    obj = Dict(red=1, green=2, blue=3)   # this object is weak referenceable
 
 Other built-in types such as :class:`tuple` and :class:`long` do not support
-weak references even when subclassed (those types implemented as a
-:ctype:`PyVarObject`).
+weak references even when subclassed (This is an implementation detail and may
+be different across various Python implementations.).
 
 Extension types can easily be made to support weak references; see
 :ref:`weakref-support`.