Clarify vehicle_id persistence rule (#632)

This PR:
- Makes the rule about vehicle_id persistence more explicit: rotation is between rentals (not every time the feed is loaded)
- Updates a forgotten reference to free_bike_status.json (now vehicle_status.json)

Thanks @testower for flagging!

gbfs.md

@@ -205,7 +205,7 @@ Example: The `rental_methods` field contains values `creditcard`, `paypass`, etc
     * Does not have to be globally unique, unless otherwise specified
     * MUST be in the ASCII printable character range, space excluded (0x21 to 0x7E) https://en.wikipedia.org/wiki/ASCII#Printable_characters *(as of v3.0)*
     * SHOULD be restricted to `A-Z`, `a-z`, `0-9` and `.@:/_-` *(as of v3.0)*
-    * MUST be persistent for a given entity (station, plan, etc.). An exception is `vehicle_id`, which MUST NOT be persistent for privacy reasons (see `vehicle_status.json`). *(as of v2.0)*
+    * MUST be persistent for a given entity (station, plan, etc.). An exception is `vehicle_id`, which MUST NOT be persistent between rentals for privacy reasons (see `vehicle_status.json`). *(as of v2.0)*
 * Language - An IETF BCP 47 language code. For an introduction to IETF BCP 47, refer to https://www.rfc-editor.org/rfc/bcp/bcp47.txt and https://www.w3.org/International/articles/language-tags/. Examples: `en` for English, `en-US` for American English, or `de` for German.
 * Latitude - WGS84 latitude in decimal degrees. The value MUST be greater than or equal to -90.0 and less than or equal to 90.0. Example: `41.890169` for the Colosseum in Rome.
 * Longitude - WGS84 longitude in decimal degrees. The value MUST be greater than or equal to -180.0 and less than or equal to 180.0. Example: `12.492269` for the Colosseum in Rome.
@@ -546,7 +546,7 @@ Field Name | REQUIRED | Type | Defines
 `vehicle_types[].max_permitted_speed`<br/>*(added in v2.3)* | OPTIONAL | Non-negative Integer | The maximum speed in kilometers per hour this vehicle is permitted to reach in accordance with local permit and regulations.
 `vehicle_types[].rated_power`<br/>*(added in v2.3)* | OPTIONAL | Non-negative Integer | The rated power of the motor for this vehicle type in watts.
 `vehicle_types[].default_reserve_time`<br/>*(added in v2.3)* | 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 `vehicle_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. 
-`vehicle_types[].return_constraint`<br/>*(as of v2.3)*| 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 the vehicle 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_types[].return_constraint`<br/>*(as of v2.3)*| 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 the vehicle 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 `vehicle_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_types[].vehicle_assets`<br/>*(added in v2.3)*| OPTIONAL | Object | Object containing the branding information for this vehicle type.
 `vehicle_types[].vehicle_assets.icon_url`<br/>*(added in v2.3)*| REQUIRED | URL | 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.
 `vehicle_types[].vehicle_assets.icon_url_dark`<br/>*(added in v2.3)*| 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.