[3.13] gh-120567: Clarify weekday return in calendar.monthrange docstring (GH-120570) (#120597)

gh-120567: Clarify weekday return in calendar.monthrange docstring (GH-120570)
(cherry picked from commit bd4516d9efee109dd3b02a3d60845f9053fc6718)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Hugo van Kemenade <1324225+hugovk@users.noreply.github.com>