[ACPI] document cmdline acpi_os_name=

This can sometimes be used to work around broken BIOS.
Use "Microsoft Windows" to take the same path
through the BIOS as Windows98 would.

The default is "Microsoft Windows NT", which
is what NT and later versions of Windows use,
and is the most tested path through most BIOS.

Set it to anything else, including "Linux", at your
own risk, as it seems that virtually no BIOS
has been tested with anything but the two options above.

Note that this uses the legacy _OS interface, so
we don't expect this to ever change.

Signed-off-by: Len Brown <len.brown@intel.com>