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>