[3.13] gh-128409: Document that pdb.post_mortem() accepts exceptions … (#128767)

* [3.13] gh-128409: Document that pdb.post_mortem() accepts exceptions (GH-128410)
(cherry picked from commit 1b39b502d33c68f52fd775c4e6c2174baddd40bd)

Co-authored-by: Adam Johnson <me@adamj.eu>
Co-authored-by: Stan Ulbrych <89152624+StanFromIreland@users.noreply.github.com>