sched/wait: Document timeout corner case

The timeout may elapse without 0 being returned, such as when waiting
on an unused queue. Document this possibility.

Signed-off-by: Scot Doyle <lkml14@scotdoyle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/alpine.LNX.2.11.1408241710070.6462@localhost.localdomain


Signed-off-by: Ingo Molnar <mingo@kernel.org>