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

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

Fixes: ca91ea34 ("asus-wmi: Add panel overdrive functionality")
Signed-off-by: Luke D. Jones <luke@ljones.dev>
Link: https://lore.kernel.org/r/20220812222509.292692-4-luke@ljones.dev


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