workqueue: Better document teardown for delayed_work

destroy_workqueue() does not ensure that non-pending work submitted with
queue_delayed_work() gets cancelled. The caller has to ensure that
manually.

Add this information about delayed_work in destroy_workqueue()'s
docstring.

Add a TODO for destroy_workqueue() to wait for all delayed_work.

Signed-off-by: Philipp Stanner <phasta@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>