Fix CONSTRAINTS_DOC formatting in modeling docs

This is one possible fix for #2119.
(sorry, I meant to attach commits to that issue)

It looks like this:

![screen shot 2014-02-22 at 10 40 42](https://f.cloud.github.com/assets/852409/2237404/3f0658cc-9ba6-11e3-847b-29955a544592.png)