gitcli: document that command line trumps config and env

gitcli: document that command line trumps config and env

We centrally explain that "--no-whatever" is the way to countermand
the "--whatever" option.  Explain that a configured default and the
value specified by an environment variable can be overridden by the
corresponding command line option, too.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Acked-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>