[3.14] gh-144257: document return values of PyModule_SetDocString (GH-144258) (GH-144263)

gh-144257: document return values of PyModule_SetDocString (GH-144258)
(cherry picked from commit 3e9a5b022f01fa95b4485fcf2c0c87bfb9fff837)

Co-authored-by: Nathan Goldbaum <nathan.goldbaum@gmail.com>
Co-authored-by: sobolevn <mail@sobolevn.me>