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>

Documentation/git-config.txt

@@ -71,6 +71,7 @@ codes are:
 
 On success, the command returns the exit code 0.
 
+[[OPTIONS]]
 OPTIONS
 -------
 
@@ -331,11 +332,9 @@ All writing options will per default write to the repository specific
 configuration file. Note that this also affects options like `--replace-all`
 and `--unset`. *'git config' will only ever change one file at a time*.
 
-You can override these rules either by command-line options or by environment
+You can override these rules using the `--global`, `--system`,
-variables. The `--global`, `--system` and `--worktree` options will limit
+`--local`, `--worktree`, and `--file` command-line options; see
-the file used to the global, system-wide or per-worktree file respectively.
+<<OPTIONS>> above.
-The `GIT_CONFIG` environment variable has a similar effect, but you
-can specify any filename you want.
 
 
 ENVIRONMENT