doc/git-config: clarify GIT_CONFIG environment variable

The scope and utility of the GIT_CONFIG variable was drastically reduced by dc87183189 (Only use GIT_CONFIG in "git config", not other programs, 2008-06-30). But the documentation in git-config(1) predates that, which makes it rather misleading. These days it is really just another way to say "--file". So let's say that, and explicitly make it clear that it does not impact other Git commands (like GIT_CONFIG_SYSTEM, etc, would). I also bumped it to the bottom of the list of variables, and warned people off of using it. We don't have any plans for deprecation at this point, but there's little point in encouraging people to use it by putting it at the top of the list. Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>