bpo-28617 Fixed docs inaccuracies about the types that support membership tests (GH-9086)

<!-- issue-number: [bpo-28617](https://www.bugs.python.org/issue28617) -->
https://bugs.python.org/issue28617
<!-- /issue-number -->

Doc/library/stdtypes.rst

@@ -197,8 +197,8 @@ exception.
    operator: not in
 
 Two more operations with the same syntactic priority, :keyword:`in` and
-:keyword:`not in`, are supported only by sequence types (below).
-
+:keyword:`not in`, are supported by types that are :term:`iterable` or
+implement the :meth:`__contains__` method.
 
 .. _typesnumeric:
 

Misc/NEWS.d/next/Documentation/2018-09-06-22-39-47.bpo-28617.MjnJLz.rst

+Fixed info in the stdtypes docs concerning the types that support membership
+tests.