Clarify from/to_stop_id desc in transfers.txt

gtfs/spec/en/reference.md

@@ -625,8 +625,8 @@ For a given ordered pair of arriving trip and departing trip, the transfer with
 
 |  Field Name | Type | Presence | Description |
 |  ------ | ------ | ------ | ------ |
-|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop or station where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. Refering to a station is forbiden for `transfer_types` 4 and 5. |
-|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Identifies a stop or station where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. Refering to a station is forbiden for `transfer_types` 4 and 5. |
+|  `from_stop_id` | Foreign ID referencing `stops.stop_id` | **Required** | Identifies a stop (location_type=0) or station (location_type=1) where a connection between routes begins. If this field refers to a station, the transfer rule applies to all its child stops. It must refer to a stop if `transfer_type` is 4 or 5. |
+|  `to_stop_id` | Foreign ID referencing `stops.stop_id` | **Required** | Identifies a stop (location_type=0) or station (location_type=1) where a connection between routes ends. If this field refers to a station, the transfer rule applies to all child stops. It must refer to a stop if `transfer_type` is 4 or 5. |
 | `from_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection begins.<br><br>If `from_route_id` is defined, the transfer will apply to the arriving trip on the route for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence. |
 | `to_route_id`  | Foreign ID referencing `routes.route_id` | Optional | Identifies a route where a connection ends.<br><br>If `to_route_id` is defined, the transfer will apply to the departing trip on the route for the given `to_stop_id`.<br><br>If both `to_trip_id` and `to_route_id` are defined, the `trip_id` must belong to the `route_id`, and `to_trip_id` will take precedence. |
 | `from_trip_id`  | Foreign ID referencing `trips.trip_id` | **Conditionally Required** | Identifies a trip where a connection between routes begins.<br><br>If `from_trip_id` is defined, the transfer will apply to the arriving trip for the given `from_stop_id`.<br><br>If both `from_trip_id` and `from_route_id` are defined, the `trip_id` must belong to the `route_id`, and `from_trip_id` will take precedence. REQUIRED if `transfer_type` is `4` or `5`. |