Add explicit summary of goals.
Start on fallback descriptions.

This is a light adaption of the concepts Brad describes in [1]. I'm
unconvinced that this is something really valuable, so I'm not going
to do more than stub it out for discussion. :)

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2012Nov/0129.html
Add a short example to the introduction.
Regenerate draft.