kernel/workqueue: Document wq_worker_last_func() argument

This patch avoids that the following warning is reported when building
with W=1:

kernel/workqueue.c:938: warning: Function parameter or member 'task' not described in 'wq_worker_last_func'

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Tejun Heo <tj@kernel.org>