Sync .proto and reference files

gtfs-realtime/proto/gtfs-realtime.proto

@@ -1080,6 +1080,7 @@ message Stop {
 
 // NOTE: This field is still experimental, and subject to change. It may be formally adopted in the future.
 message TripModifications {
+  // A `Modification` message replaces a span of n stop times from each affected trip starting at `start_stop_selector`.
   message Modification {
     // The stop selector of the first stop_time of the original trip that is to be affected by this modification.
     // Used in conjuction with `end_stop_selector`. 

gtfs-realtime/spec/en/reference.md

@@ -642,7 +642,7 @@ _Propagated detour delays affect all stops following the end of a modification.
 | _**Field Name**_ | _**Type**_ | _**Required**_ | _**Cardinality**_ | _**Description**_ |
 |------------------|------------|----------------|-------------------|-------------------|
 | **start_stop_selector** | [StopSelector](#message-stopselector) | Required | One | The stop selector of the first stop of the original trip that is to be affected by this modification. Used in conjuction with `end_stop_selector`. `start_stop_selector` is required and is used to define the reference stop used with `travel_time_to_stop`. See [`travel_time_to_stop`](#message-replacementstop) for details |
-| **end_stop_selector** | [StopSelector](#message-stopselector) | Conditionally required | One | The stop selector of the last stop of the original trip that is to be affected by this modification. The selection is inclusive, so if only one stop_time is replaced by that modification, `start_stop_selector` and `end_stop_selector` should be equivalent. If no stop_time is replaced, `end_stop_selector` should not be provided. It's otherwise required.  |
+| **end_stop_selector** | [StopSelector](#message-stopselector) | Conditionally required | One | The stop selector of the last stop of the original trip that is to be affected by this modification. The selection is inclusive, so if only one stop_time is replaced by that modification, `start_stop_selector` and `end_stop_selector` must be equivalent. If no stop_time is replaced, `end_stop_selector` must not be provided. It's otherwise required.  |
 | **propagated_modification_delay** | [int32](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | The number of seconds of delay to add to all departure and arrival times subsequent to the last stop inserted by a modification. If a modification affects only the shape (i.e. neither `end_stop_selector` nor `replacement_stops` are provided), then the delay propagation begins at the subsequent stop after `start_stop_selector`. Can be a positive or negative number. If multiple modifications apply to the same trip, the delays accumulate as the trip advances. <br/><br/>If the value is not supplied, consumers MAY interpolate or infer the `propagated_modification_delay` based on other data.  |
 | **replacement_stops** | [ReplacementStop](#message-replacementstop) | Optional | Many | A list of replacement stops, replacing those of the original trip. The length of the new stop times may be less, the same, or greater than the number of replaced stop times. |
 | **service_alert_id** | [string](https://protobuf.dev/programming-guides/proto2/#scalar) | Optional | One | An `id` value from the `FeedEntity` message that contains the `Alert` describing this Modification for user-facing communication. |