Update FORWARD_COLLISION_WARNING_ENABLED documentation in HAL

     * Set true to enable FCW and false to disable FCW. When FCW is enabled, the ADAS system in the

     * vehicle should be turned on and monitoring for potential collisions.

     *

     * If FCW is not available, IVehicle#get must not return any NOT_AVAILABLE value in StatusCode.

     * Other StatusCode values like TRY_AGAIN may still be used as needed. For example, if FCW is

     * not available because the vehicle speed is too low, IVehicle#get must return false.

     * In general, FORWARD_COLLISION_WARNING_ENABLED should always return true or false. If the

     * feature is not available due to some temporary state, such as the vehicle speed being too

     * low, that information must be conveyed through the ErrorState values in the

     * FORWARD_COLLISION_WARNING_STATE property.

     *

     * This property is defined as read_write, but OEMs have the option to implement it as read

     * only.