ACPI: ibm_acpi: Documentation the wan feature.

Document the wan feature Jeremy Fitzhardinge added to ibm_acpi.

Signed-off-by: Stefan Schmidt <stefan@datenfreihafen.org>
Acked-by: Borislav Deianov <borislav@users.sourceforge.net>
Signed-off-by: Len Brown <len.brown@intel.com>