platform/x86: asus-wmi: Document the dgpu_disable sysfs attribute

The dgpu_disable attribute was not documented, this adds the
required documentation.

Fixes: 98829e84 ("asus-wmi: Add dgpu disable method")
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20220812222509.292692-2-luke@ljones.devReviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>

Documentation/ABI/testing/sysfs-platform-asus-wmi

@@ -57,3 +57,12 @@ Description:
 			* 0 - default,
 			* 1 - overboost,
 			* 2 - silent
+
+What:		/sys/devices/platform/<platform>/dgpu_disable
+Date:		Aug 2022
+KernelVersion:	5.17
+Contact:	"Luke Jones" <luke@ljones.dev>
+Description:
+		Disable discrete GPU:
+			* 0 - Enable dGPU,
+			* 1 - Disable dGPU