I added the 'Q' to list. A short description in the `Ok, so what can I
use them for'-section, on when or why to use it would be nice!

Signed-off-by: Johannes Weiner <hannes-kernel@saeurebad.de>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>