sched, kernel-doc: Fix runqueue_is_locked() description

Fix kernel-doc warning for runqueue_is_locked():

  Warning(kernel/sched.c:664): missing initial short description

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <20110315161230.c4e1e8e3.rdunlap@xenotime.net>
Signed-off-by: Ingo Molnar <mingo@elte.hu>