sched: Fix yield_to kernel-doc

Add missing function parameters for yield_to():

 Warning(kernel/sched.c:5470): No description found for parameter 'p'
 Warning(kernel/sched.c:5470): No description found for parameter 'preempt'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Peter Zijlstra <peterz@infradead.org>
LKML-Reference: <20110318093453.8f7489a4.randy.dunlap@oracle.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>