doc: mention approxidates for git-commit --date

doc: mention approxidates for git-commit --date

We describe the more strict date formats accepted by GIT_COMMITTER_DATE,
etc, but the --date option also allows the looser approxidate formats,
as well. Unfortunately we don't have a good or complete reference for
this format, but let's at least mention that it _is_ looser, and give a
few examples.

If we ever write separate, more complete date-format documentation, we
should refer to it from here.

Based-on-a-patch-by: Utku Gultopu <ugultopu@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>