doc: add a few missing @ref tags

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
touchpad: print the device name on error

A little bit easier to recognize.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Drop unused libinput_device->terminated field

Unused since commit 56f7dde "Port udev-seat to be used in libinput"

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>