AP_GPS: update save_cfg description

Saving config is not expected to work on a LEA-6H based on user reports, so the parameter should warn of this

libraries/AP_GPS/AP_GPS.cpp

@@ -104,7 +104,7 @@ const AP_Param::GroupInfo AP_GPS::var_info[] = {
 
     // @Param: SAVE_CFG
     // @DisplayName: Save GPS configuration
-    // @Description: Determines whether the configuration for this GPS should be written to non-volatile memory on the GPS. Currently working for UBlox.
+    // @Description: Determines whether the configuration for this GPS should be written to non-volatile memory on the GPS. Currently working for UBlox 7 series and above.
     // @Values: 0:Do not save config,1:Save config
     // @User: Advanced
     AP_GROUPINFO("SAVE_CFG", 11, AP_GPS, _save_config, 0),