[3.12] Document that return-less user-defined functions return None (GH-126769) (#126823)

Document that return-less user-defined functions return None (GH-126769)
(cherry picked from commit e0692f11650acb6c2eed940eb94650b4703c072e)

Co-authored-by: John Marshall <jmarshall@hey.com>
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>