sched, docs: Update schedstats documentation to version 15

Version 15 of schedstats was introduced in:

   67aa0f76: sched: remove unused fields from struct rq

and removed three unused counters in sched_yield(). Update
the documentation.

Signed-off-by: Javi Merino <cibervicho@gmail.com>
Cc: henrix@sapo.pt
Cc: rdunlap@xenotime.net
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <1296515496-8229-1-git-send-email-cibervicho@gmail.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>