sched/completion: Document when to use wait_for_completion_io_*()
sched/completion: Document when to use wait_for_completion_io_*()

As discussed in [1], accounting IO is meant for blkio only. Document that
so driver authors won't use them for device io.

 [1] http://thread.gmane.org/gmane.linux.drivers.i2c/20470

Signed-off-by: Wolfram Sang <fd4ce474653598159cad06f3c83387a05cd53a44@the-dreams.de>
Signed-off-by: Peter Zijlstra (Intel) <3fddac958924aef220f202ca567388ddab3f14a8@infradead.org>
Cc: One Thousand Gnomes <71add150d99de1bd5a1ceb2e3464b76ba775ecb5@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <69652caca27c8b940640ad396ab71f93cacec34f@linux-foundation.org>
Link: http://lkml.kernel.org/r/1415098901-2768-1-git-send-email-fd4ce474653598159cad06f3c83387a05cd53a44@the-dreams.de
Signed-off-by: Ingo Molnar <9dbbbf0688fedc85ad4da37637f1a64b8c718ee2@kernel.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs