docs: driver-api: Add Surface DTX driver documentation

Add documentation for the user-space interface of the Surface DTX
(detachment system) driver, used on Microsoft Surface Book series
devices.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210308184819.437438-4-luzmaximilian@gmail.com


Signed-off-by: Hans de Goede <hdegoede@redhat.com>