cxl: Document missing sysfs files

Add to the ABI documentation the payload_max and label_storage_size
read-only files, which have been there since the early days.

Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Reviewed-by: Li Ming <ming.li@zohomail.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20250218224853.67457-3-dave@stgolabs.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Davidlohr Bueso

Documentation/ABI/testing/sysfs-bus-cxl

@@ -18,6 +18,24 @@ Description:
 		specification.
 
 
+What:		/sys/bus/cxl/devices/memX/payload_max
+Date:		December, 2020
+KernelVersion:	v5.12
+Contact:	linux-cxl@vger.kernel.org
+Description:
+		(RO) Maximum size (in bytes) of the mailbox command payload
+		registers. Linux caps this at 1MB if the device reports a
+		larger size.
+
+
+What:		/sys/bus/cxl/devices/memX/label_storage_size
+Date:		May, 2021
+KernelVersion:	v5.13
+Contact:	linux-cxl@vger.kernel.org
+Description:
+		(RO) Size (in bytes) of the Label Storage Area (LSA).
+
+
 What:		/sys/bus/cxl/devices/memX/ram/size
 Date:		December, 2020
 KernelVersion:	v5.12