git-rev-parse.txt: clarify --git-dir

git-rev-parse.txt: clarify --git-dir

The current behavior is often to print an absolute path rather than
a ../../etc string, but callers must be ready to accept a relative
path, too. The most common output is ".git" (from the toplevel of
an ordinary work tree).

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>