sched: Add comments for wd_start_next and work_queue_next.

This commit added comments for newly added APIs wd_start_next and
work_queue_next.

Signed-off-by: ouyangxiangzhen <ouyangxiangzhen@xiaomi.com>

include/nuttx/wdog.h

@@ -294,7 +294,11 @@ int wd_start_realtime(FAR struct wdog_s *wdog,
  *
  * Description:
  *   This function restart watchdog timer based on the last expiration time.
- *   It can be used to implement a periodic watchdog timer.
+ *   It can be used to implement a periodic watchdog timer. E.g, Call this
+ *   function instead of wd_start in the watchdog callback to restart the
+ *   next timer for better timing accuracy.
+ *   Note that calling this function outside the watchdog callback requires
+ *   the wdog->expired being set.
  *
  * Input Parameters:
  *   wdog     - Pointer of the periodic watchdog.

include/nuttx/wqueue.h

@@ -414,7 +414,11 @@ int work_queue_wq(FAR struct kwork_wqueue_s *wqueue,
  *
  * Description:
  *   Queue work to be performed at a later time based on the last expiration
- *   time. This function must be called in the workqueue callback.
+ *   time. This function can be used to implement a periodic workqueue.
+ *   E.g, Call this function instead of work_queue in the work callback to
+ *   restart the next work for better timing accuracy.
+ *   Note that calling this function outside the work callback requires
+ *   the work->qtime being set.
  *
  * Input Parameters:
  *   qid    - The work queue ID (must be HPWORK or LPWORK)