Plane: expand the VTAIL_OUTPUT docs for APM1

ArduPlane/Parameters.pde

@@ -500,7 +500,7 @@ const AP_Param::Info var_info[] PROGMEM = {
 
     // @Param: VTAIL_OUTPUT
     // @DisplayName: VTail output
-    // @Description: Enable VTail output in software. If enabled then the APM will provide software VTail mixing on the elevator and rudder channels. There are 4 different mixing modes available, which refer to the 4 ways the elevator can be mapped to the two VTail servos. Note that you must not use VTail output mixing with hardware pass-through of RC values, such as with channel 8 manual control on an APM1.
+    // @Description: Enable VTail output in software. If enabled then the APM will provide software VTail mixing on the elevator and rudder channels. There are 4 different mixing modes available, which refer to the 4 ways the elevator can be mapped to the two VTail servos. Note that you must not use VTail output mixing with hardware pass-through of RC values, such as with channel 8 manual control on an APM1. So if you use an APM1 then set FLTMODE_CH to something other than 8 before you enable VTAIL_OUTPUT.
     // @Values: 0:Disabled,1:UpUp,2:UpDown,3:DownUp,4:DownDown
     // @User: User
     GSCALAR(vtail_output,           "VTAIL_OUTPUT",  0),