workqueue: Upgrade queue_work_on() comment

The current queue_work_on() docbook comment says that the caller must
ensure that the specified CPU can't go away, but does not spell out the
consequences, which turn out to be quite mild.  Therefore expand this
comment to explicitly say that the penalty for failing to nail down the
specified CPU is that the workqueue handler might find itself executing
on some other CPU.

Cc: Tejun Heo <tj@kernel.org>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>