bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383)

Doc/reference/datamodel.rst

@@ -2155,8 +2155,10 @@ through the container; for mappings, :meth:`__iter__` should be the same as
 
    Called to implement :func:`operator.length_hint`. Should return an estimated
    length for the object (which may be greater or less than the actual length).
-   The length must be an integer ``>=`` 0. This method is purely an
-   optimization and is never required for correctness.
+   The length must be an integer ``>=`` 0. The return value may also be
+   *NotImplemented*, which is treated the same as if the ``__length_hint__``
+   method didn't exist at all. This method is purely an optimization and is
+   never required for correctness.
 
    .. versionadded:: 3.4