[PATCH] Fix comment to synchronize_sched()

Fix to broken comment to synchronize_rcu() noted by Keith Owens.  Also add
sentence noting that synchronize_sched() and synchronize_rcu() are not
necessarily identical.

Signed-off-by: Paul E. McKenney <paulmck@us.ibm.com>
Cc: Keith Owens <kaos@sgi.com>
Cc: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>