bpo-37913: Link to NotImplemented from new docs (pythonGH-15860)

Author: zooba

Doc/reference/datamodel.rst

@@ -2156,9 +2156,9 @@ 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. 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.
+   :const:`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