bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383)
diff --git a/Doc/reference/datamodel.rst b/Doc/reference/datamodel.rst
index 8813f57..c6c6e40 100644
--- a/Doc/reference/datamodel.rst
+++ b/Doc/reference/datamodel.rst
@@ -2155,7 +2155,9 @@
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
+ The length must be an integer ``>=`` 0. The return value may also be
+ :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