docs: reword MQTT_HOME_ASSISTANT_DISCOVERY_URL wording

MQTT_HOME_ASSISTANT_DISCOVERY_URL is optional; clarify that setting MQTT_HOME_ASSISTANT_DISCOVERY does not require it. Addresses review comment on #5543.

Author: brianmay

website/docs/configuration/environment_variables.md

@@ -35,7 +35,7 @@ TeslaMate accepts the following environment variables for runtime configuration:
 | **MQTT_TLS_ACCEPT_INVALID_CERTS**        | Accepts invalid certificates if `true`                                                                                                                                                                                                                                                                                 | false                                                                                                                                                               |
 | **MQTT_IPV6**                            | Set to `true` if IPv6 should be used                                                                                                                                                                                                                                                                                   | false                                                                                                                                                               |
 | **MQTT_NAMESPACE**                       | Inserts a custom namespace into the MQTT topic. For example, with `MQTT_NAMESPACE=account_0`: `teslamate/account_0/cars/$car_id/state`.                                                                                                                                                                                |                                                                                                                                                                     |
-| **MQTT_HOME_ASSISTANT_DISCOVERY**        | Enables Home Assistant MQTT discovery if `true`. TeslaMate publishes discovery config payloads to `<MQTT_HOME_ASSISTANT_DISCOVERY_PREFIX>/.../config` so entities appear in Home Assistant automatically, removing the need for the manual `mqtt_sensors.yaml`. Set together with `MQTT_HOME_ASSISTANT_DISCOVERY_URL`. | false                                                                                                                                                               |
+| **MQTT_HOME_ASSISTANT_DISCOVERY**        | Enables Home Assistant MQTT discovery if `true`. TeslaMate publishes discovery config payloads to `<MQTT_HOME_ASSISTANT_DISCOVERY_PREFIX>/.../config` so entities appear in Home Assistant automatically, removing the need for the manual `mqtt_sensors.yaml`. Optionally set together with `MQTT_HOME_ASSISTANT_DISCOVERY_URL`. | false                                                                                                                                                               |
 | **MQTT_HOME_ASSISTANT_DISCOVERY_URL**    | Optional: the TeslaMate URL surfaced in the discovered device panel (e.g. `https://teslamate.example.com/`). Only used when `MQTT_HOME_ASSISTANT_DISCOVERY` is `true`.                                                                                                                                                 |                                                                                                                                                                     |
 | **MQTT_HOME_ASSISTANT_DISCOVERY_PREFIX** | Discovery topic prefix. Matches Home Assistant's MQTT discovery prefix.                                                                                                                                                                                                                                                | homeassistant                                                                                                                                                       |
 | **IMPORT_DIR**                           | The path of the directory for the import of data (e.g. TeslaFi)                                                                                                                                                                                                                                                        | ./import                                                                                                                                                            |