Documentation/scheduler/sched-deadline.txt: Improve and clarify AC bits

Documentation/scheduler/sched-deadline.txt: Improve and clarify AC bits

Admission control is of key importance for SCHED_DEADLINE, since
it guarantees system schedulability (or tells us something about
the degree of guarantees we can provide to the user).

This patch improves and clarifies bits and pieces regarding AC,
both for UP and SMP systems.

Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Juri Lelli <juri.lelli@arm.com>
Reviewed-by: Henrik Austad <henrik@austad.us>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dario Faggioli <raistlin@linux.it>
Cc: Juri Lelli <juri.lelli@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/1410256636-26171-4-git-send-email-juri.lelli@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>