RC_Channel: rename 'Manual' to RCPassThru

this should make it clearer what it does

libraries/RC_Channel/RC_Channel_aux.cpp

@@ -12,7 +12,7 @@ const AP_Param::GroupInfo RC_Channel_aux::var_info[] PROGMEM = {
     // @Param: FUNCTION
     // @DisplayName: Servo out function
     // @Description: Setting this to Disabled(0) will disable this output, any other value will enable the corresponding function
-    // @Values: 0:Disabled,1:Manual,2:Flap,3:Flap_auto,4:Aileron,5:flaperon,6:mount_pan,7:mount_tilt,8:mount_roll,9:mount_open,10:camera_trigger,11:release,12:mount2_pan,13:mount2_tilt,14:mount2_roll,15:mount2_open,16:DifferentialSpoiler1,17:DifferentialSpoiler2,18:AileronWithInput,19:Elevator,20:ElevatorWithInput,21:Rudder
+    // @Values: 0:Disabled,1:RCPassThru,2:Flap,3:Flap_auto,4:Aileron,5:flaperon,6:mount_pan,7:mount_tilt,8:mount_roll,9:mount_open,10:camera_trigger,11:release,12:mount2_pan,13:mount2_tilt,14:mount2_roll,15:mount2_open,16:DifferentialSpoiler1,17:DifferentialSpoiler2,18:AileronWithInput,19:Elevator,20:ElevatorWithInput,21:Rudder
     // @User: Standard
     AP_GROUPINFO("FUNCTION",       1, RC_Channel_aux, function, 0),