[3.11] gh-98930: improve the docstring of signal.strsignal (GH-99290) (#99449)

Improves the docstring on signal.strsignal to make it explain when it returns a message, None, or when it raises ValueError.

Closes GH-98930

Co-authored-by: Gregory P. Smith <greg@krypto.org>.
(cherry picked from commit 88385b856490a5ce3c99160bde6050638f81f72e)

Co-authored-by: ram vikram singh <ramvikrams243@gmail.com>