date: document and test "raw-local" mode

The "raw" format shows a Unix epoch timestamp, but with a timezone tacked on. The timestamp is not _in_ that zone, but it is extra information about the time (by default, the zone the author was in). The documentation claims that "raw-local" does not work. It does, but the end result is rather subtle. Let's describe it in better detail, and test to make sure it works (namely, the epoch time doesn't change, but the zone does). While we are rewording the documentation in this area, let's not use the phrase "does not work" for the remaining option, "--date=relative". It's vague; do we accept it or not? We do accept it, but it has no effect (which is a reasonable outcome). We should also refer to the option not as "--relative" (which is the historical synonym, and does not take "-local" at all), but as "--date=relative". Helped-by: Jakub Narębski <jnareb@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>