Update Virtual USB Ethernet protocol visibility in qtdeviceutilities documentation

Describe how to enable/disable the virtual USB Ethernet protocol menu

Preconditions for this are that configuration file..

\badcode
/etc/default/qdbd
\endcode

..exists and that the following property from \e{viewSettings.qml} file is set true.

\badcode
property bool usbEthernetSettingVisible: true
\endcode

If preconditions are not met, the menu \uicontrol{Settings} > \uicontrol{Network}
looks like this:

\image b2qt-network-usb-ethernet-setting-hidden.png