rev-parse doc: --git-dir does not always show a relative path

The description was misleading because it lead the reader to believe
that --git-dir would always show a relative path when, in fact, the
actual behaviour does not guarantee this.

Rather, it was intended that the advice be given that if a relative
path is shown, then the path is relative to the current working
directory and not some other directory (for example, the root of the
working tree).

Signed-off-by: Jon Seymour <jon.seymour@gmail.com>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-rev-parse.txt

@@ -137,7 +137,8 @@ appending `/{asterisk}`.
 
 --git-dir::
 	Show `$GIT_DIR` if defined. Otherwise show the path to
-	the .git directory, relative to the current directory.
+	the .git directory. The path shown, when relative, is
+	relative to the current working directory.
 +
 If `$GIT_DIR` is not defined and the current directory
 is not detected to lie in a git repository or work tree