message: action_impl: better comment about custom modes
diff: @@ -49,6 +49,8 @@ private:
  static constexpr uint8_t VEHICLE_MODE_FLAG_CUSTOM_MODE_ENABLED = 1;
  // FIXME: these chould potentially change anytime
  + // derived from:
  + // https://github.com/PX4/Firmware/blob/master/src/modules/commander/px4_custom_mode.h
  static constexpr uint8_t PX4_CUSTOM_MAIN_MODE_AUTO = 4;
  static constexpr uint8_t PX4_CUSTOM_SUB_MODE_AUTO_RTL = 5;
  };
  