rfkill: Update userspace API documentation

Add a note to userspace on the effect of RFKILL_OP_CHANGE_ALL also
updating the default state for hotplugged devices.
Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
[reword a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/uapi/linux/rfkill.h

@@ -59,6 +59,8 @@ enum rfkill_type {
  * @RFKILL_OP_DEL: a device was removed
  * @RFKILL_OP_CHANGE: a device's state changed -- userspace changes one device
  * @RFKILL_OP_CHANGE_ALL: userspace changes all devices (of a type, or all)
+ *	into a state, also updating the default state used for devices that
+ *	are hot-plugged later.
  */
 enum rfkill_operation {
 	RFKILL_OP_ADD = 0,