Documentation/git-for-each-ref: don't list date formats

We are about to add a new set of supported date formats and do not want
to have to maintain the same list in several different bits of
documentation.  Refer to git-rev-list(1) which contains the full list of
supported formats.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-for-each-ref.txt

@@ -134,9 +134,8 @@ the object referred by the ref does not cause an error.  It
 returns an empty string instead.
 
 As a special case for the date-type fields, you may specify a format for
-the date by adding one of `:default`, `:relative`, `:short`, `:local`,
-`:iso8601`, `:rfc2822` or `:raw` to the end of the fieldname; e.g.
-`%(taggerdate:relative)`.
+the date by adding `:` followed by date format name (see the
+values the `--date` option to linkgit::git-rev-list[1] takes).
 
 
 EXAMPLES