platform/x86: wmi: Add bus ABI documentation

Add documentation for the WMI bus sysfs interface so userspace
applications can use it to access additional data about WMI devices.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Armin Wolf <W_Armin@gmx.de>
Link: https://lore.kernel.org/r/20240624173116.31314-3-W_Armin@gmx.de


Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>