vm.txt: document min_free_pages as critical for correctness

min_free_pages is critical for correctness, document it as such.

Signed-off-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>