Extend VLAN documentation (drop policy, etc)

[jovatn]

We could add more info on VLAN behavior, for instance regarding isolation.

E.g., consider a VLAN bridge where e0 has PVID 0 and is configured untagged to VLAN 10.

• Incoming packet with VID 20: Thrown away
• Incoming packet with VID 0: Priority tagged. Accepted and mapped to VID 10, using packet PCP
• Incoming packet with VID 10: Accepted. Treated just as the priority tagged packet

Are there differences between SW bridges and bridges with HW offloaded? I believe the behavior above apply both for SW bridges and when offloaded on Marvell Linkstreet.