Add best practice: stop_headsign does not "carry down"

gtfs/spec/en/reference.md

@@ -200,7 +200,7 @@ File: **Required**
 |  `departure_time` | Time | **Conditionally required** | Departure time from a specific stop for a specific trip on a route. For times occurring after midnight on the service day, enter the time as a value greater than 24:00:00 in HH:MM:SS local time for the day on which the trip schedule begins. If there are not separate times for arrival and departure at a stop, enter the same value for `arrival_time` and `departure_time`. See the `arrival_time` description for more details about using timepoints correctly. |
 |  `stop_id` | ID referencing `stops.stop_id` | **Required** | Identifies the serviced stop. All stops serviced during a trip must have a record in [stop_times.txt](#stop_timestxt). Referenced locations must be stops, not stations or station entrances. A stop may be serviced multiple times in the same trip, and multiple trips and routes may service the same stop. |
 |  `stop_sequence` | Non-negative integer | **Required** | Order of stops for a particular trip. The values must increase along the trip but do not need to be consecutive.<hr>*Example: The first location on the trip could have a `stop_sequence`=`1`, the second location on the trip could have a `stop_sequence`=`23`, the third location could have a `stop_sequence`=`40`, and so on.* |
-|  `stop_headsign` | Text | Optional | Text that appears on signage identifying the trip's destination to riders. This field overrides the default `trips.trip_headsign` when the headsign changes between stops. If the headsign is displayed for an entire trip, use `trips.trip_headsign` instead. |
+|  `stop_headsign` | Text | Optional | Text that appears on signage identifying the trip's destination to riders. This field overrides the default `trips.trip_headsign` when the headsign changes between stops. If the headsign is displayed for an entire trip, use `trips.trip_headsign` instead. <br><br>  A `stop_headsign` value specified for one `stop_time` does not apply to subsequent `stop_time`s in the same trip. If you want to override the `trip_headsign` for multiple `stop_time`s in the same trip, the `stop_headsign` value must be repeated in each `stop_time` row. |
 |  `pickup_type` | Enum | Optional | Indicates pickup method. Valid options are:<br><br>`0` or empty - Regularly scheduled pickup. <br>`1` - No pickup available.<br>`2` - Must phone agency to arrange pickup.<br>`3` - Must coordinate with driver to arrange pickup. |
 |  `drop_off_type` | Enum | Optional | Indicates drop off method. Valid options are:<br><br>`0` or empty - Regularly scheduled drop off.<br>`1` - No drop off available.<br>`2` - Must phone agency to arrange drop off.<br>`3` - Must coordinate with driver to arrange drop off. |
 |  `shape_dist_traveled` | Non-negative float | Optional | Actual distance traveled along the associated shape, from the first stop to the stop specified in this record. This field specifies how much of the shape to draw between any two stops during a trip. Must be in the same units used in [shapes.txt](#shapestxt). Values used for `shape_dist_traveled` must increase along with `stop_sequence`; they cannot be used to show reverse travel along a route.<hr>*Example: If a bus travels a distance of 5.25 kilometers from the start of the shape to the stop,`shape_dist_traveled`=`5.25`.*|