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>

kernel/workqueue.c

@@ -913,6 +913,7 @@ struct task_struct *wq_worker_sleeping(struct task_struct *task)
 
 /**
  * wq_worker_last_func - retrieve worker's last work function
+ * @task: Task to retrieve last work function of.
  *
  * Determine the last function a worker executed. This is called from
  * the scheduler to get a worker's last known identity.