[3.13] Highlight `datetime.timedelta.seconds` vs `.total_seconds()` in docs. (GH-124811) (#124862)

Highlight `datetime.timedelta.seconds` vs `.total_seconds()` in docs. (GH-124811)

Thanks to the reviewers for suggesting the use of a "caution" section instead of "warning" or "note".
(cherry picked from commit d150e4abcfc13770c2d239878ed337fb53e51de5)

Co-authored-by: Gregory P. Smith <greg@krypto.org>