gh-141510: Document isinstance change for frozendict in What's New (#146107)

Author: vstinner

Doc/whatsnew/3.15.rst

@@ -221,6 +221,11 @@ The following standard library modules have been updated to accept
 :func:`eval` and :func:`exec` accept :class:`!frozendict` for *globals*, and
 :func:`type` and :meth:`str.maketrans` accept :class:`!frozendict` for *dict*.
 
+Code checking for :class:`dict` type using ``isinstance(arg, dict)`` can be
+updated to ``isinstance(arg, (dict, frozendict))`` to accept also the
+:class:`!frozendict` type, or to ``isinstance(arg, collections.abc.Mapping)``
+to accept also other mapping types such as :class:`~types.MappingProxyType`.
+
 .. seealso:: :pep:`814` for the full specification and rationale.
 
 (Contributed by Victor Stinner and Donghee Na in :gh:`141510`.)