ACPI: property: Document the _DSD data buffer GUID

As the rest of the GUIDs document one for _DSD data buffer
so it will be eaisier to search for on internet or documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>