message: action: improve docs for goto altitude
  Currently, PX4 does not let us specifiy absolute altitudes for goto, so
  AMSL (above mean sea level) is all we have.
diff: @@ -138,7 +138,10 @@ public:
  Result return_to_launch() const;
  /**
  - * @brief Send command to reposition the vehicle to a specific WGS84 global position
  + * @brief Send command to reposition the vehicle to a specific position.
  + *
  + * The latitude and longitude is given in degrees in WGS84 and the altitude
  + * in meters AMSL (above mean sea level).
  *
  * This sends the vehicle to a specified lattitude/longitude/altitude coordinates.
  * @param latitude_deg Latitude in degrees
  