doc/git-config: clarify GIT_CONFIG environment variable

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>