LanItems documentation.

Similarly to https://github.com/yast/yast-yast2/pull/271, I needed to find this out while fixing bnc#883836 (WIP).
http://www.rubydoc.info/github/yast/yast-network/Yast/LanItemsClass will have the new docs once it is merged.
To see it locally, run `yardoc` and `xdg-open doc/autodocs/Yast/LanItemsClass.html`

I used YARD macros to document the horde of attributes
I think it should be together with other attributes