advice: recommend GIT_ADVICE=0 for tools

advice: recommend GIT_ADVICE=0 for tools

The GIT_ADVICE environment variable was added implicitly in b79deeb5544
(advice: add --no-advice global option, 2024-05-03) but was not
documented. Add documentation to show that it is an option for tools
that want to disable these messages. Make note that while the
--no-advice option exists, older Git versions will fail to parse that
option. The environment variable presents a way to change the behavior
of Git versions that understand it without disrupting older versions.

Co-authored-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>