vm: document that setting vfs_cache_pressure to 0 isn't a good idea

Reported-by: Christian Thaeter <ct@pipapo.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>