platform/x86: dell-ddv: Add documentation

The WMI interface used by the dell-wmi-ddv driver contains
many methods which are currently unused, making it difficult
to document these inside the drivers source code.
Create the necessary documentation based on current knowledge
so that all details of the WMI interface can be written down
for later use.
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20230508204241.11076-1-W_Armin@gmx.deReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

MAINTAINERS

@@ -5828,6 +5828,7 @@ M:	Armin Wolf <W_Armin@gmx.de>
 S:	Maintained
 F:	Documentation/ABI/testing/debugfs-dell-wmi-ddv
 F:	Documentation/ABI/testing/sysfs-platform-dell-wmi-ddv
+F:	Documentation/wmi/devices/dell-wmi-ddv.rst
 F:	drivers/platform/x86/dell/dell-wmi-ddv.c
 
 DELL WMI SYSMAN DRIVER