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

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

Fixes: 382b91db ("asus-wmi: Add egpu enable method")
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20220812222509.292692-3-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

@@ -66,3 +66,14 @@ Description:
 		Disable discrete GPU:
 			* 0 - Enable dGPU,
 			* 1 - Disable dGPU
+
+What:		/sys/devices/platform/<platform>/egpu_enable
+Date:		Aug 2022
+KernelVersion:	5.17
+Contact:	"Luke Jones" <luke@ljones.dev>
+Description:
+		Enable the external GPU paired with ROG X-Flow laptops.
+		Toggling this setting will also trigger ACPI to disable the dGPU:
+
+			* 0 - Disable,
+			* 1 - Enable