doc/git-config: simplify "override" advice for FILES section

doc/git-config: simplify "override" advice for FILES section

At the end of the FILES section, we indicate that you can override the
regular lookup rules with --global, etc. But:

  - we're missing the --local option

  - we point to GIT_CONFIG instead of --file, but the latter has much
    better documentation

  - we're vague about how the overrides work; the actual option
    descriptions are much better here

So let's just mention the names and point people back to the OPTIONS
section. We could perhaps even delete this paragraph entirely, but the
presence of the names may give people reading FILES a clue about where
to look for more information.

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