Clarify stops.zone_id conditional requirement for route-based fare_ru…

…les (#432)

* Clarify stops.zone_id conditional requirement for route-based fare_rules

* Change stops.zone_id to optional

gtfs/spec/en/reference.md

@@ -207,7 +207,7 @@ Primary key (`stop_id`)
 |  `stop_desc` | Text | Optional | Description of the location that provides useful, quality information. Should not be a duplicate of `stop_name`.|
 |  `stop_lat` | Latitude | **Conditionally Required** | Latitude of the location.<br><br>For stops/platforms (`location_type=0`) and boarding area (`location_type=4`), the coordinates must be the ones of the bus pole — if exists — and otherwise of where the travelers are boarding the vehicle (on the sidewalk or the platform, and not on the roadway or the track where the vehicle stops). <br><br>Conditionally Required:<br>- **Required** for locations which are stops (`location_type=0`), stations (`location_type=1`) or entrances/exits (`location_type=2`).<br>- Optional for locations which are generic nodes (`location_type=3`) or boarding areas (`location_type=4`).|
 |  `stop_lon` | Longitude | **Conditionally Required** | Longitude of the location.<br><br>For stops/platforms (`location_type=0`) and boarding area (`location_type=4`), the coordinates must be the ones of the bus pole — if exists — and otherwise of where the travelers are boarding the vehicle (on the sidewalk or the platform, and not on the roadway or the track where the vehicle stops). <br><br>Conditionally Required:<br>- **Required** for locations which are stops (`location_type=0`), stations (`location_type=1`) or entrances/exits (`location_type=2`).<br>- Optional for locations which are generic nodes (`location_type=3`) or boarding areas (`location_type=4`). |
-|  `zone_id` | ID | **Conditionally Required** | Identifies the fare zone for a stop. If this record represents a station or station entrance, the `zone_id` is ignored.<br><br>Conditionally Required:<br>- **Required** if providing fare information using [fare_rules.txt](#fare_rulestxt) <br>- Optional otherwise.|
+|  `zone_id` | ID | Optional | Identifies the fare zone for a stop. If this record represents a station or station entrance, the `zone_id` is ignored.|
 |  `stop_url` | URL | Optional | URL of a web page about the location. This should be different from the `agency.agency_url` and the `routes.route_url` field values. |
 |  `location_type` | Enum | Optional | Location type. Valid options are:<br><br>`0` (or blank) - **Stop** (or **Platform**). A location where passengers board or disembark from a transit vehicle. Is called a platform when defined within a `parent_station`.<br>`1` - **Station**. A physical structure or area that contains one or more platform.<br>`2` - **Entrance/Exit**. A location where passengers can enter or exit a station from the street. If an entrance/exit belongs to multiple stations, it may be linked by pathways to both, but the data provider must pick one of them as parent.<br>`3` - **Generic Node**. A location within a station, not matching any other `location_type`, that may be used to link together pathways define in [pathways.txt](#pathwaystxt).<br>`4` - **Boarding Area**. A specific location on a platform, where passengers can board and/or alight vehicles.|
 |  `parent_station` | Foreign ID referencing `stops.stop_id` | **Conditionally Required** | Defines hierarchy between the different locations defined in [stops.txt](#stopstxt). It contains the ID of the parent location, as followed:<br><br>- **Stop/platform** (`location_type=0`): the `parent_station` field contains the ID of a station.<br>- **Station** (`location_type=1`): this field must be empty.<br>- **Entrance/exit** (`location_type=2`) or **generic node** (`location_type=3`): the `parent_station` field contains the ID of a station (`location_type=1`)<br>- **Boarding Area** (`location_type=4`): the `parent_station` field contains ID of a platform.<br><br>Conditionally Required:<br>- **Required** for locations which are entrances (`location_type=2`), generic nodes (`location_type=3`) or boarding areas (`location_type=4`).<br>- Optional for stops/platforms (`location_type=0`).<br>- Forbidden for stations (`location_type=1`).|