[dox] add some more comments on used altitude definitions

sw/airborne/state.h

@@ -227,6 +227,7 @@ struct State {
    * Defines the origin of the local NorthEastDown coordinate system
    * in UTM coordinates, used as a reference when ned_origin is not
    * initialized.
+   * Altitude is height above MSL.
    * (float version)
    */
   struct UtmCoor_f utm_origin_f;

sw/airborne/subsystems/navigation/common_nav.h

@@ -44,6 +44,7 @@ struct point {
 
 #define WaypointX(_wp) (waypoints[_wp].x)
 #define WaypointY(_wp) (waypoints[_wp].y)
+/** waypoint altitude in m above MSL */
 #define WaypointAlt(_wp) (waypoints[_wp].a)
 #define Height(_h) (_h + ground_alt)
 
@@ -53,7 +54,8 @@ extern const uint8_t nb_waypoint;
 extern struct point waypoints[];
 /** size == nb_waypoint, waypoint 0 is a dummy waypoint */
 
-extern float ground_alt; /* m */
+/** altitude of the ground in m above MSL */
+extern float ground_alt;
 
 extern int32_t nav_utm_east0;  /* m */
 extern int32_t nav_utm_north0; /* m */