Update directive.ngdoc

Organizes list of possible directive names into a `<ul>`.

## Before
![screen shot 2013-10-17 at 1 40 47 pm](https://f.cloud.github.com/assets/420959/1354861/bd7ebae6-375b-11e3-9ad3-e7ed75338312.png)

## After
![screen shot 2013-10-17 at 1 40 37 pm](https://f.cloud.github.com/assets/420959/1354862/c3164e10-375b-11e3-97a2-1f7ec1657d66.png)