sched: doc: added docs to the scheduling subsystem

All of the scheduling core, the threading API and the event subystem are
now commented using doxygen.

Signed-off-by: Michel Megens <dev@michelmegens.net>
sched: doc: added docs to the rr scheduler

* Added doxygen style code comments to rr.c - the backend source of the
  round robin scheduler.

* Updated the documentation on the sched_class structure.

Signed-off-by: Michel Megens <dev@michelmegens.net>