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>

Documentation/date-formats.txt

@@ -1,10 +1,7 @@
 DATE FORMATS
 ------------
 
-The `GIT_AUTHOR_DATE`, `GIT_COMMITTER_DATE` environment variables
-ifdef::git-commit[]
-and the `--date` option
-endif::git-commit[]
+The `GIT_AUTHOR_DATE` and `GIT_COMMITTER_DATE` environment variables
 support the following date formats:
 
 Git internal format::
@@ -26,3 +23,9 @@ ISO 8601::
 +
 NOTE: In addition, the date part is accepted in the following formats:
 `YYYY.MM.DD`, `MM/DD/YYYY` and `DD.MM.YYYY`.
+
+ifdef::git-commit[]
+In addition to recognizing all date formats above, the `--date` option
+will also try to make sense of other, more human-centric date formats,
+such as relative dates like "yesterday" or "last Friday at noon".
+endif::git-commit[]