net: qmi_wwan: document qmap/mux_id sysfs file

Document qmap/mux_id sysfs file showing qmimux interface id
Signed-off-by: Daniele Palmas <dnlplm@gmail.com>
Acked-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/ABI/testing/sysfs-class-net-qmi

@@ -48,3 +48,13 @@ Description:
 
 		Write a number ranging from 1 to 254 to delete a previously
 		created qmap mux based network device.
+
+What:		/sys/class/net/<qmimux iface>/qmap/mux_id
+Date:		January 2021
+KernelVersion:	5.12
+Contact:	Daniele Palmas <dnlplm@gmail.com>
+Description:
+		Unsigned integer
+
+		Indicates the mux id associated to the qmimux network interface
+		during its creation.