Generalize a comment which was VLAN-specific.

The explanation in this comment was based around the VLAN network type.
Granted, right now we don't have any others, but this should be phrased
in a way that doesn't break abstraction boundaries. The `Network` class
should know nothing about VLANs (and it doesn't).
Merge pull request #311 from zenhack/generalize-comment

Generalize a comment which was VLAN-specific.