git-status.txt: mention --no-optional-locks

git-status.txt: mention --no-optional-locks

If you come to the documentation thinking "I do not want Git
to take any locks for my background processes", then you may
easily run across "--no-optional-locks" in git.txt.

But it's quite reasonable to hit a specific instance of the
problem: you have "git status" running in the background,
and you notice that it causes lock contention with other
processes. So you look in git-status.txt to see if there is
a way to disable it, but there's no mention of the flag.

Let's add a short note mentioning that status does indeed
touch the index (and why), with a pointer to the global
option. That can point users in the right direction and help
them make a more informed decision about what they're
disabling.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>