ACPI: Document _OSI and _REV for Linux BIOS writers

Based on a recent session at the Linux Plumber's Conference,
we need to be more clear about how a BIOS should use _OSI
to properly support Linux.

Signed-off-by: Len Brown <len.brown@intel.com>
Reviewed-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>