[3.13] gh-148663: Document that `calendar.IllegalMonthError` inherits from both `ValueError` and `IndexError` (GH-148664) (#148918)

(cherry picked from commit 435be06dd25a5e4e19014340c4ba873d71051c4c)

Co-authored-by: Eoin Shaughnessy <45000144+EoinTrial@users.noreply.github.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>
Co-authored-by: Stan Ulbrych <stan@python.org>