workqueue: update comments and a warning message

* Update incorrect and add missing synchronization labels.

* Update incorrect or misleading comments.  Add new comments where
  clarification is necessary.  Reformat / rephrase some comments.

* drain_workqueue() can be used separately from destroy_workqueue()
  but its warning message was incorrectly referring to destruction.

Other than the warning message change, this patch doesn't make any
functional changes.

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