message: added brief documentation for overloaded template
diff: namespace mavsdk {
  +/**
  + * @brief Creates an overloaded callable object from multiple other callables
  + */
  template<class... Ts> struct overloaded : Ts... {
  using Ts::operator()...;
  };
  +/**
  + * @brief Template deduction helper for `overloaded`
  + */
  template<class... Ts> overloaded(Ts...) -> overloaded<Ts...>;
  /**
  