date: clarify --date=raw description

"... in the internal raw Git format `%s %z` format." was clunky in
repeating "format" twice, and would not have helped those who do not
immediately get that these are strftime(3) conversion specifiers.

Explain them with words, and demote the mention of `%s %z` to a
hint to help those who know them.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

Documentation/rev-list-options.txt

@@ -746,7 +746,11 @@ format, often found in email messages.
 +
 `--date=short` shows only the date, but not the time, in `YYYY-MM-DD` format.
 +
-`--date=raw` shows the date in the internal raw Git format `%s %z` format.
+`--date=raw` shows the date as seconds since the epoch (1970-01-01
+00:00:00 UTC), followed by a space, and then the timezone as an offset
+from UTC (a `+` or `-` with four digits; the first two are hours, and
+the second two are minutes). I.e., as if the timestamp were formatted
+with `strftime("%s %z")`).
 Note that the `-local` option does not affect the seconds-since-epoch
 value (which is always measured in UTC), but does switch the accompanying
 timezone value.