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>