Link keywords in documentation to their associated docs

- [x] `_.*where` -> `_.matches`
- [x] `iteratee` -> `_.iteratee`

[**Rendered**](http://rawgit.com/megawac/underscore/iteratee-docv/index.html)
It's a good question but in general I'm not a fan of overloading the method
signature docs like this. I'd say leave it as keys then add a note in the
body about the alternative args (since the keys is the common behavior)