Documentation: ABI: add FBPT and S3PT entries to sysfs-firmware-acpi

Document the FBPT and S3PT sysfs attributes located under
/sys/firmware/acpi/fpdt/.

Signed-off-by: Nate DeSimone <nathaniel.l.desimone@intel.com>
[ rjw: Changelog tweak ]
Link: https://patch.msgid.link/20260324231456.701-3-nathaniel.l.desimone@intel.com
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Author: nate-desimone

Documentation/ABI/testing/sysfs-firmware-acpi

@@ -41,6 +41,12 @@ Description:
 				platform runtime firmware S3 resume, just prior to
 				handoff to the OS waking vector. In nanoseconds.
 
+		FBPT: The raw binary contents of the Firmware Basic Boot
+			Performance Table (FBPT) subtable.
+
+		S3PT: The raw binary contents of the S3 Performance Table
+			(S3PT) subtable.
+
 What:		/sys/firmware/acpi/bgrt/
 Date:		January 2012
 Contact:	Matthew Garrett <mjg@redhat.com>