events: document special values of time in zone_events_schedule_at

src/knot/events/events.h

@@ -100,12 +100,18 @@ void zone_events_enqueue(struct zone *zone, zone_event_type_t type);
 /*!
  * \brief Schedule new zone event.
  *
- * If the event is already scheduled, the new time will be set only if the
- * new time is earlier than the currently scheduled one. An exception is
- * a zero time, which causes event cancellation.
- *
  * The function allows to set multiple events at once.
  *
+ * The function intreprets time values (t) as follows:
+ *
+ *   t > 0: schedule timer for a given time
+ *   t = 0: cancel the timer
+ *   t < 0: ignore change in the timer
+ *
+ * If the event is already scheduled, the new time will be set only if the
+ * new time is earlier than the currently scheduled one. To override the
+ * check, cancel and schedule the event in a single function call.
+ *
  * \param zone  Zone to schedule new event for.
  * \param ...   Sequence of zone_event_type_t and time_t terminated with
  *              ZONE_EVENT_INVALID.