futex: update documentation for ordering guarantees

futex: update documentation for ordering guarantees

Commits ed7bb8891725 ("futex: revert back to the explicit waiter
counting code") and 18bcfdf36204 ("futex: avoid race between requeue and
wake") changed some of the finer details of how we think about futexes.
One was a late fix and the other a consequence of overlooking the whole
requeuing logic.

The first change caused our documentation to be incorrect, and the
second made us aware that we need to explicitly add more details to it.

Signed-off-by: Davidlohr Bueso <davidlohr@hp.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>