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

commit 7e64c486 upstream.

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.dev


Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>