config doc: remove confusion about relative GIT_DIR from FILES section

From the FILES section of the git-config(1) manual:

	$GIT_DIR/config::
		Repository specific configuration file. (The filename is
		of course relative to the repository root, not the working
		directory.)

That's confusing because $GIT_DIR really is relative to the working
directory.

	$ GIT_DIR=.git GIT_EDITOR='pwd; echo editing'
	$ export GIT_DIR GIT_EDITOR
	$ git config --edit --local
	/home/jrn/src/git/Documentation
	editing .git/config

It turns out that the comment is a remnant from older days when the
heading said ".git/config" (which is indeed relative to the top of the
worktree).

It was only when the heading was changed to refer more precisely to
<git dir>/config (see v1.5.3.2~18, AsciiDoc tweak to avoid leading
dot, 2007-09-14) that the parenthesis stopped making sense.  Remove
it.
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-config.txt

@@ -198,9 +198,7 @@ If not set explicitly with '--file', there are three files where
 'git config' will search for configuration options:
 
 $GIT_DIR/config::
-	Repository specific configuration file. (The filename is
-	of course relative to the repository root, not the working
-	directory.)
+	Repository specific configuration file.
 
 ~/.gitconfig::
 	User-specific configuration file. Also called "global"