Update documentation.

Author: serhiy-storchaka

Doc/c-api/number.rst

@@ -196,8 +196,9 @@ Number Protocol
 
    See the built-in function :func:`pow`. Returns ``NULL`` on failure.  The operation
    is done *in-place* when *o1* supports it.  This is the equivalent of the Python
-   statement ``o1 **= o2`` when o3 is :c:data:`Py_None`, or an in-place variant of
-   ``pow(o1, o2, o3)`` otherwise. If *o3* is to be ignored, pass :c:data:`Py_None`
+   statement ``o1 **= o2`` when o3 is :c:data:`Py_None`, or hypothetical in-place variant of
+   ``pow(o1, o2, o3)`` (not supported in Python) otherwise.
+   If *o3* is to be ignored, pass :c:data:`Py_None`
    in its place (passing ``NULL`` for *o3* would cause an illegal memory access).
 
 

Doc/reference/datamodel.rst

@@ -3398,6 +3398,13 @@ left undefined.
    :ref:`faq-augmented-assignment-tuple-error`), but this behavior is in fact
    part of the data model.
 
+   Note that :meth:`__ipow__` should be defined to accept an optional third
+   argument if the third non-``None`` argument of
+   :c:func:`PyNumber_InPlacePower` is to be supported.
+
+   .. versionchanged:: next
+      :meth:`!__ipow__` can now be called with three arguments in
+      :c:func:`!PyNumber_InPlacePower`.
 
 .. method:: object.__neg__(self)
             object.__pos__(self)

Doc/whatsnew/3.14.rst

@@ -307,6 +307,12 @@ Other language changes
   The testbed can also be used to run the test suite of projects other than
   CPython itself. (Contributed by Russell Keith-Magee in :gh:`127592`.)
 
+* The :meth:`~object.__ipow__` method should be defined to accept an optional
+  third argument (as it was documented) to support non-``None`` third argument
+  in :c:func:`PyNumber_InPlacePower` which no longer ignored if
+  :meth:`!__ipow__` is defined.
+  (Contributed by Serhiy Storchaka in :gh:`86069`.)
+
 New modules
 ===========