sched/dl/Documentation: Clarify the relationship between tasks' deadlines and absolute scheduling deadlines

Clarify what is the relationship between tasks' parameters and scheduling
parameters, explaining how to set the scheduling parameters so that all the
absolute deadlines of a task are respected.
Signed-off-by: Luca Abeni <luca.abeni@unitn.it>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: henrik@austad.us
Cc: juri.lelli@gmail.com
Cc: raistlin@linux.it
Link: http://lkml.kernel.org/r/1431954032-16473-9-git-send-email-luca.abeni@unitn.itSigned-off-by: Ingo Molnar <mingo@kernel.org>