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>

kernel/sched/completion.c

@@ -148,7 +148,7 @@ EXPORT_SYMBOL(wait_for_completion_timeout);
  *
  * This waits to be signaled for completion of a specific task. It is NOT
  * interruptible and there is no timeout. The caller is accounted as waiting
- * for IO.
+ * for IO (which traditionally means blkio only).
  */
 void __sched wait_for_completion_io(struct completion *x)
 {
@@ -163,7 +163,8 @@ EXPORT_SYMBOL(wait_for_completion_io);
  *
  * This waits for either a completion of a specific task to be signaled or for a
  * specified timeout to expire. The timeout is in jiffies. It is not
- * interruptible. The caller is accounted as waiting for IO.
+ * interruptible. The caller is accounted as waiting for IO (which traditionally
+ * means blkio only).
  *
  * Return: 0 if timed out, and positive (at least 1, or number of jiffies left
  * till timeout) if completed.