Swconfig: enable_vlan and enable_vlan4k documentation

[lleachii]

@znark, I saw this linked in another thread - hope this helps and clears confusion on what the value means:

DSA and Primary VLAN and local?

Pvid is only need in special async vlan setups (where ingress tagging != egress tagging). You usually do not need to specify it explicitly. It either implicitly defaults to the first untagged VLAN configured on the port or it is not defined at all if unspecified.