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gh-104219: Document that idunders can return NotImplemented (#104220) 

Co-authored-by: C.A.M. Gerlach <CAM.Gerlach@Gerlach.CAM>
Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>
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Doc/reference/datamodel.rst
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@ -3044,10 +3044,12 @@ 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