Clarify the documentation of QinQ flags, and extend the meaning of the
flag: if PKT_RX_QINQ_STRIPPED is set and PKT_RX_VLAN_STRIPPED is unset,
only the outer VLAN is removed from packet data, but both tci are saved
in mbuf->vlan_tci (inner) and mbuf->vlan_tci_outer (outer).