Add list of supported media types for pull request review comments

This fixes a broken link in the docs page for [Pull Request Review Comments](http://developer.github.com/v3/pulls/comments/) by adding the list of supported media types at the bottom of the page:

> Pull Request Review Comments use [these custom media types](http://developer.github.com/v3/pulls/comments/#custom-media-types) _(this was the broken link)_. You can read more about the use of media types in the API [here](http://developer.github.com/v3/media/).

Pull request review comments support the same media types as issue comments.