Rover config: Add comments and set correct output group

ROMFS/px4fmu_common/init.d/rc.rover_defaults

@@ -2,22 +2,27 @@
 
 set VEHICLE_TYPE rover
 
-# i dont think AUTOCNF is needed here, can it be removed?
-
+# This section can be enabled once tuning parameters for this particular
+# rover model are known. It allows to configure default gains via the GUI
 #if [ $AUTOCNF == yes ]
 #then
 #	# param set MC_ROLL_P 7.0
 #fi
 
-#PWM Hz
+# PWM Hz - 50 Hz is the normal rate in RC cars, higher rates
+# may damage analog servos.
 set PWM_RATE 50
 
-#PWW default value for "disarmed" mode
+# PWM default value for "disarmed" mode
+# this centers the steering and throttle, which means no motion
+# for a rover
 set PWM_DISARMED 1500
 
-#PWM range
+# PWM range
 set PWM_MIN 1200
 set PWM_MAX 1800
 
-#enable servo output on pins 3 and 4 (steering and thrust)
-set PWM_OUT 34
+# Enable servo output on pins 3 and 4 (steering and thrust)
+# but also include 1+2 as they form together one output group
+# and need to be set together.
+set PWM_OUT 1234