Document the responsibility of NetworkInterfaces.

I needed to find this out while fixing [bnc#883836](https://bugzilla.novell.com/show_bug.cgi?id=883836) (WIP).
http://www.rubydoc.info/github/yast/yast-yast2/Yast/NetworkInterfacesClass will have the new docs once it is merged.
To see it locally, run `yardoc` and `xdg-open doc/autodocs/Yast/NetworkInterfacesClass.html`

Notice how to document YCP variables. Comments inside `main` wouldn't work.