virtio_net: document VIRTIO_NET_CTRL_GUEST_OFFLOADS

Document VIRTIO_NET_CTRL_GUEST_OFFLOADS and the
relevant feature bits.

Will allow ethtool control of the offloads down the road.

Reported-by: Yan Vugenfirer <yan@daynix.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>