git: update documentation for --git-dir

git: update documentation for --git-dir

git --git-dir <path> is a bit confusing and sometimes doesn't work as
the user would expect it to.

For example, if the user runs `git --git-dir=<path> status`, git
will skip the repository discovery algorithm and will assign the
work tree to the user's current work directory unless otherwise
specified. When this assignment is wrong, the output will not match
the user's expectations.

This patch updates the documentation to make it clearer.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>