AP_Follow: add FOLL log message documentation

libraries/AP_Follow/AP_Follow.cpp

@@ -322,6 +322,18 @@ void AP_Follow::handle_msg(const mavlink_message_t &msg)
         }
 
         // log lead's estimated vs reported position
+// @LoggerMessage: FOLL
+// @Description: Follow library diagnostic data
+// @Field: TimeUS: Time since system startup
+// @Field: Lat: Target latitude
+// @Field: Lon: Target longitude
+// @Field: Alt: Target absolute altitude
+// @Field: VelN: Target earth-frame velocity, North
+// @Field: VelE: Target earth-frame velocity, East
+// @Field: VelD: Target earth-frame velocity, Down
+// @Field: LatE: Vehicle latitude
+// @Field: LonE: Vehicle longitude
+// @Field: AltE: Vehicle absolute altitude
         AP::logger().Write("FOLL",
                                                "TimeUS,Lat,Lon,Alt,VelN,VelE,VelD,LatE,LonE,AltE",  // labels
                                                "sDUmnnnDUm",    // units