update return_constraint

Updates return_constraint to match master

gbfs.md

@@ -438,7 +438,7 @@ Field Name | REQUIRED | Type | Defines
 \- `max_permitted_speed`<br/>*(added in v2.3-RC2)* | OPTIONAL | Non-negative Integer | The maximum speed in kilometers per hour this vehicle is permitted to reach in accordance with local permit and regulations.
 \- `rated_power`<br/>*(added in v2.3-RC2)* | OPTIONAL | Non-negative Integer | The rated power of the motor for this vehicle type in watts.
 \- `default_reserve_time`<br/>*(added in v2.3-RC)* | OPTIONAL | Non-negative Integer | Maximum time in minutes that a vehicle can be reserved before a rental begins. When a vehicle is reserved by a user, the vehicle remains locked until the rental begins. During this time the vehicle is unavailable and cannot be reserved or rented by other users. The vehicle status in `free_bike_status.json` MUST be set to `is_reserved = true`. If the value of `default_reserve_time` elapses without a rental beginning, the vehicle status MUST change to `is_reserved = false`. If `default_reserve_time` is set to `0`, the vehicle type cannot be reserved. 
-\- `return_type`<br/>*(added in v2.3-RC)*| OPTIONAL | Array | The conditions for returning the vehicle at the end of the trip. For vehicles that have more than one return option, include all applicable methods in the array. <br /><br />Current valid values are:<br /><ul><li>`free_floating` _(The vehicle can be returned anywhere permitted within the service area.  Note that this field is subject to rules in `geofencing_zones.json` if defined.)_</li><li>`roundtrip_station` _(The vehicle must be returned to the initial rental station. Cannot be defined in combination with `free_floating`.)_</li><li>`any_station` _(The vehicle must be returned to any station within the service area . Note that a specific station can be defined in `free_bike_status.json` using `home_station`. Cannot be defined in combination with `roundtrip_station`.)_
+\- `return_constraint`<br/>*(as of v2.3-RC2)*| OPTIONAL | Enum | The conditions for returning the vehicle at the end of the rental.<br /><br />Current valid values are:<br /><ul><li>`free_floating` _(The vehicle can be returned anywhere permitted within the service area.  Note that this field is subject to rules in `geofencing_zones.json` if defined.)_</li><li>`roundtrip_station` _(The vehicle has to be returned to the same station from which it was initially rented. Note that a specific station can be assigned to the vehicle in `free_bike_status.json` using home_station.)_</li><li>`any_station` _(The vehicle has to be returned to any station within the service area .)_</li><li>`hybrid` (The vehicle can be returned to any station, or anywhere else permitted within the service area. Note that the vehicle is subject to rules in `geofencing_zones.json` if defined.)</li>
 \- `vehicle_assets`<br/>*(added in v2.3-RC)*| OPTIONAL | Object | An object where each key defines one of the items listed below.
 &emsp;&emsp;\- `icon_url`<br/>*(added in v2.3-RC)*| Conditionally REQUIRED | URL | REQUIRED if `vehicle_assets` is defined. A fully qualified URL pointing to the location of a graphic icon file that MAY be used to represent this vehicle type on maps and in other applications. File MUST be in SVG V1.1 format and MUST be either square or round.
 &emsp;&emsp;\- `icon_url_dark`<br/>*(added in v2.3-RC)*| OPTIONAL | URL | A fully qualified URL pointing to the location of a graphic icon file to be used to represent this vehicle type when in dark mode on maps and in other applications. File MUST be in SVG V1.1 format and MUST be either square or round.