DOC: more pd.to_datetime examples
xref #12996

@jorisvandenbossche 
 also added #12974 fix here.
 @jreback Can you please try to not change the commit too much during the merge? This makes reviewing PRs / seeing what has been merged a lot more difficult using github (as it seems the 'Files changed' is quite different from the actual merged commit).

(of course, small fixup changes are not a problem, certainly for PRs of other contributors to not have them endlessly adapt minor things)
 @jorisvandenbossche I normally don't. but ended up going on a tangent with some other things (somewhat unrelated).
 That's OK! (I just noticed it for this one)
