Documentation: ACPI: Use all-string data node references

Documentation: ACPI: Use all-string data node references

Document that references to data nodes shall use string-only references
instead of a device reference and a succession of the first package
entries of hierarchical data node references.

Fixes: 9880702d123f ("ACPI: property: Support using strings in reference properties")
Cc: 6.8+ <stable@vger.kernel.org> # 6.8+
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Link: https://patch.msgid.link/20250409084738.3657079-1-sakari.ailus@linux.intel.com
[ rjw: Clarifying edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>