Associating input devices with outputs is undocumented
Weston allows associating input devices with specific outputs by using
libinput_device_get_output_name(). However, libinput documentation says that function is discouraged and does not explain where the value actually comes from. Weston documentation fails to explain it as well.
The value comes from the udev property
WL_OUTPUT on the input device.
Document this in Weston so that users can find it.