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

Documentation/driver-api/surface_aggregator/clients/index.rst

@@ -11,6 +11,7 @@ This is the documentation for client drivers themselves. Refer to
    :maxdepth: 1
 
    cdev
+   dtx
    san
 
 .. only::  subproject and html

MAINTAINERS

@@ -11865,6 +11865,7 @@ MICROSOFT SURFACE DTX DRIVER
 M:	Maximilian Luz <luzmaximilian@gmail.com>
 L:	platform-driver-x86@vger.kernel.org
 S:	Maintained
+F:	Documentation/driver-api/surface_aggregator/clients/dtx.rst
 F:	drivers/platform/surface/surface_dtx.c
 F:	include/uapi/linux/surface_aggregator/dtx.h