[RFC PATCH] doc: clarify VLAN and QinQ stripping behaviour

[Morten Brørup]

FYI:
Last time I looked, it seemed like the VLAN/QINQ "tag present" mbuf RX flags were only set along with the RX _STRIPPED flags, i.e. not set when stripping was not enabled.
Although setting the "tag present" flags (and the vlan_tci/vlan_tci_outer fields) without stripping would be nice to have, I think we should initially focus on stripping behavior.