virtio-ccw: document GPR4/GPR2 cookie values

virtio-ccw on kvm can use a cookie value passed to guest
to optimize channel/VQ lookups.
Document this.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au> (s/s390/ccw/)