workqueue: Clarify that schedule_on_each_cpu is synchronous

The documentation for schedule_on_each_cpu() states that it calls a
function on each online CPU from keventd.  This can easily be
interpreted as an asyncronous call because the description does not
mention that flush_work is called.  Clarify that it is synchronous.

tj: rephrased a bit

Signed-off-by: Mel Gorman <mel@csn.ul.ie>
Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Signed-off-by: Tejun Heo <tj@kernel.org>