Further 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, and further says that the
penalty for failing to nail down the specified CPU is that the workqueue
handler might find itself executing on some other CPU.  This is true
as far as it goes, but fails to note what happens if the specified CPU
never was online.  Therefore, further expand this comment to say that
specifying a CPU that was never online will result in a splat.

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