doc/git-config: explain --file instead of referring to GIT_CONFIG

doc/git-config: explain --file instead of referring to GIT_CONFIG

The explanation for the --file option only refers to GIT_CONFIG. This
redirection to an environment variable is confusing, but doubly so
because the description of GIT_CONFIG is out of date.

Let's describe --file from scratch, detailing both the reading and
writing behavior as we do for other similar options like --system, etc.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>