commander arming: disable stick (dis-)arming when arming switch/button is used

A reason for using switch arming in the first place is that you cannot
accidentally disarm in-air with the sticks during acrobatic flights.

It also adds a low-throttle arming requirement for switch arming.

src/modules/sensors/rc_params.c

@@ -1556,7 +1556,10 @@ PARAM_DEFINE_INT32(RC_MAP_OFFB_SW, 0);
 PARAM_DEFINE_INT32(RC_MAP_KILL_SW, 0);
 
 /**
- * Arm switch channel
+ * Arm switch channel.
+ *
+ * Use it to arm/disarm via switch instead of default throttle stick. If this is
+ * assigned, arming and disarming via stick is disabled.
  *
  * @min 0
  * @max 18