KVM: s390: improve documentation for S390_MEM_OP

Explicitly specify the valid ranges for size and ar, and reword
buf requirements a bit.

Signed-off-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lkml.kernel.org/r/20190829124746.28665-1-cohuck@redhat.com


Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>