docs: Update s390 vfio-ap doc for ap_config sysfs attribute

A new sysfs attribute, ap_config, for the vfio_ap driver is
documented.

Signed-off-by: "Jason J. Herne" <jjherne@linux.ibm.com>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Link: https://lore.kernel.org/r/20240415152555.13152-6-jjherne@linux.ibm.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>