message: core: improve comment about communication lock
diff: @@ -96,6 +96,8 @@ public:
  Time &get_time() { return _time; };
  // This allows a plugin to lock and unlock all mavlink communication.
  + // The functionality is currently not used by a plugin included here
  + // but nevertheless there for other plugins that can be added from external.
  void lock_communication();
  void unlock_communication();
  