sched/doc: Document capacity aware scheduling

Add some documentation detailing the concepts, requirements and
implementation of capacity aware scheduling across the different scheduler
classes.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20200731192016.7484-3-valentin.schneider@arm.com