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 <wsa@the-dreams.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: One Thousand Gnomes <gnomes@lxorguk.ukuu.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1415098901-2768-1-git-send-email-wsa@the-dreams.de


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