[3.12] gh-70647: update docs to mention the datetime 1900 year default 2/29 issue...

[3.12] gh-70647: update docs to mention the datetime 1900 year default 2/29 issue (#131534)

* gh-70647: Better promote how to safely parse yearless dates in datetime.

Every four years people encounter this because it just isn't obvious.
This moves the footnote up to a note with a code example.

We'd love to change the default year value for datetime but doing
that could have other consequences for existing code.  This documented
workaround *always* works.

* doctest code within note is bad, dedent.