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>