add reference for "two-level namespacing"

[lukewagner]

... and reference it anywhere we refer to that term in the docs.

[flagxor]

Can we clarify what this issue is for?

[jfbastien]

@flagxor: #682 (comment)

[jfbastien]

@lukewagner tag :)

[lukewagner]

As the original requester, do you have a particular reference in mind? I don't see any canonical-looking Wikipedia entry or other search result. I'm fine closing this since I don't think people are stumbling over this.

[jfbastien]

OK sounds good.