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.dev


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