#7170: fix explanation about non-weakrefable builtin types.

Doc/library/weakref.rst

@@ -76,6 +76,10 @@ 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`).
+
 Extension types can easily be made to support weak references; see
 :ref:`weakref-support`.