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

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>