Documentation: ACPI: Direct references are allowed to devices only

In ACPI it is possible to make references to device objects only,
not to other objects inside a device.

In practice this means that hierarchical data extension targets
must be in parentheses to make them strings, or an ACPICA warning
will be produced.

Reported-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
[ rjw: Changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>