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>

Documentation/git-status.txt

@@ -366,6 +366,19 @@ ignored submodules you can either use the --ignore-submodules=dirty command
 line option or the 'git submodule summary' command, which shows a similar
 output but does not honor these settings.
 
+BACKGROUND REFRESH
+------------------
+
+By default, `git status` will automatically refresh the index, updating
+the cached stat information from the working tree and writing out the
+result. Writing out the updated index is an optimization that isn't
+strictly necessary (`status` computes the values for itself, but writing
+them out is just to save subsequent programs from repeating our
+computation). When `status` is run in the background, the lock held
+during the write may conflict with other simultaneous processes, causing
+them to fail. Scripts running `status` in the background should consider
+using `git --no-optional-locks status` (see linkgit:git[1] for details).
+
 SEE ALSO
 --------
 linkgit:gitignore[5]