virtio: kdoc for struct virtio_pci_modern_device

Finally following up to Simon's suggestion for some kdoc attention
on struct virtio_pci_modern_device.

Link: https://lore.kernel.org/netdev/ZE%2FQS0lnUvxFacjf@corigine.com/


Cc: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Shannon Nelson <shannon.nelson@amd.com>
Acked-by: Eugenio Pérez <eperezma@redhat.com>
Message-Id: <20230911213104.14391-1-shannon.nelson@amd.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>