AP_Airspeed: update ARSPD_AUTOCAL docs

libraries/AP_Airspeed/AP_Airspeed.cpp

@@ -105,7 +105,7 @@ const AP_Param::GroupInfo AP_Airspeed::var_info[] PROGMEM = {
 
     // @Param: AUTOCAL
     // @DisplayName: Automatic airspeed ratio calibration
-    // @Description: If this is enabled then the APM will automatically adjust the ARSPD_RATIO during flight, based upon an estimation filter using ground speed and true airspeed. The automatic calibration will save the new ratio to EEPROM every 2 minutes if it changes by more than 5%
+    // @Description: If this is enabled then the APM will automatically adjust the ARSPD_RATIO during flight, based upon an estimation filter using ground speed and true airspeed. The automatic calibration will save the new ratio to EEPROM every 2 minutes if it changes by more than 5%. This option should be enabled for a calibration flight then disabled again when calibration is complete. Leaving it enabled all the time is not recommended.
     // @User: Advanced
     AP_GROUPINFO("AUTOCAL",  5, AP_Airspeed, _autocal, 0),