[3.8] Clarify argument types in datetime docs. (GH-15459) (GH-15472)

[3.8] Clarify argument types in datetime docs. (GH-15459) (GH-15472)

"Arguments may be integers... " could be misunderstand as they also
could be strings.

New wording makes it clear that arguments have to be integers.

modified:   Doc/library/datetime.rst

Automerge-Triggered-By: @pganssle
(cherry picked from commit c5218fce02d6bd7bb343db20c8f14e8d9640783a)

Co-authored-by: Jürgen Gmach <juergen.gmach@googlemail.com>
Automerge-Triggered-By: @pganssle