devices.txt: document rfkill allocation

Document rfkill allocation.
Signed-off-by: Pavel Machek (CIP) <pavel@denx.de>

Link: https://lore.kernel.org/r/20200726075327.GA25647@duo.ucw.czSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/devices.txt

@@ -375,8 +375,9 @@
 		239 = /dev/uhid		User-space I/O driver support for HID subsystem
 		240 = /dev/userio	Serio driver testing device
 		241 = /dev/vhost-vsock	Host kernel driver for virtio vsock
+		242 = /dev/rfkill	Turning off radio transmissions (rfkill)
 
-		242-254			Reserved for local use
+		243-254			Reserved for local use
 		255			Reserved for MISC_DYNAMIC_MINOR
 
   11 char	Raw keyboard device	(Linux/SPARC only)