docs: kvm: add documentation for KVM_CAP_S390_DIAG318

Documentation for the s390 DIAGNOSE 0x318 instruction handling.

Signed-off-by: Collin Walling <walling@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/kvm/20200625150724.10021-2-walling@linux.ibm.com/


Message-Id: <20200625150724.10021-2-walling@linux.ibm.com>
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>