Fix conditional states for stop_time_update.arrival/departure

...based on stop_time_update.schedule_relationship.  See https://github.com/google/transit/blob/master/gtfs-realtime/spec/en/reference.md#enum-schedulerelationship

gtfs-realtime/spec/en/reference.md

@@ -163,8 +163,8 @@ The update is linked to a specific stop either through stop_sequence or stop_id,
 |------------------|------------|-------------------|-------------------|
 | **stop_sequence** | [uint32](https://developers.google.com/protocol-buffers/docs/proto#scalar) | conditionally required | Must be the same as in stop_times.txt in the corresponding GTFS feed.  Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty.  stop_sequence is required for trips that visit the same stop_id more than once (e.g., a loop) to disambiguate which stop the prediction is for. |
 | **stop_id** | [string](https://developers.google.com/protocol-buffers/docs/proto#scalar) | conditionally required | Must be the same as in stops.txt in the corresponding GTFS feed. Either stop_sequence or stop_id must be provided within a StopTimeUpdate - both fields cannot be empty. |
-| **arrival** | [StopTimeEvent](#message-stoptimeevent) | conditionally required | Either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. |
-| **departure** | [StopTimeEvent](#message-stoptimeevent) | conditionally required | Either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. |
+| **arrival** | [StopTimeEvent](#message-stoptimeevent) | conditionally required | If schedule_relationship is empty or SCHEDULED, either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. arrival and departure may both be empty when schedule_relationship is SKIPPED.  If schedule_relationship is NO_DATA, arrival and departure must be empty. |
+| **departure** | [StopTimeEvent](#message-stoptimeevent) | conditionally required | If schedule_relationship is empty or SCHEDULED, either arrival or departure must be provided within a StopTimeUpdate - both fields cannot be empty. arrival and departure may both be empty when schedule_relationship is SKIPPED.  If schedule_relationship is NO_DATA, arrival and departure must be empty. |
 | **schedule_relationship** | [ScheduleRelationship](#enum-schedulerelationship) | optional | The default relationship is SCHEDULED. |
 
 ## _enum_ ScheduleRelationship