virtio: kerneldocs fixes and enhancements

Fix variable names in some kerneldocs, naming in others.
Add kerneldocs for struct vring_desc and vring_interrupt.

Signed-off-by: Ricardo Cañuelo <ricardo.canuelo@collabora.com>
Message-Id: <20220810094004.1250-2-ricardo.canuelo@collabora.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>