[3.11] gh-104219: Document that idunders can return NotImplemented (GH-104220)

Doc/reference/datamodel.rst

@@ -3016,10 +3016,12 @@ left undefined.
    (``+=``, ``-=``, ``*=``, ``@=``, ``/=``, ``//=``, ``%=``, ``**=``, ``<<=``,
    ``>>=``, ``&=``, ``^=``, ``|=``).  These methods should attempt to do the
    operation in-place (modifying *self*) and return the result (which could be,
-   but does not have to be, *self*).  If a specific method is not defined, the
+   but does not have to be, *self*).  If a specific method is not defined, or if
+   that method returns :data:`NotImplemented`, the
    augmented assignment falls back to the normal methods.  For instance, if *x*
    is an instance of a class with an :meth:`__iadd__` method, ``x += y`` is
-   equivalent to ``x = x.__iadd__(y)`` . Otherwise, ``x.__add__(y)`` and
+   equivalent to ``x = x.__iadd__(y)`` . If :meth:`__iadd__` does not exist, or if ``x.__iadd__(y)``
+   returns :data:`!NotImplemented`, ``x.__add__(y)`` and
    ``y.__radd__(x)`` are considered, as with the evaluation of ``x + y``. In
    certain situations, augmented assignment can result in unexpected errors (see
    :ref:`faq-augmented-assignment-tuple-error`), but this behavior is in fact