jekyll: List of collections not rendering

I am trying to build a list of the collections I have defined in my _config.yml, thus:

collections:
  articles:
    output: true
    title: Articles
  essays:
    output: true
    title: Academic Essays
  presentations:
    output: true
    title: Presentations
  projects:
    output: true
    title: Projects

using the following Liquid template:

{% for collection in site.collections %}
    <section>
        <h1><a href="{{ collection.directory }}">{{ collection.title }}</a></h1>
        Label: {{ collection.label }}
    </section>
{% endfor %}

but the following is rendered:

<section>
    <h1><a href=""></a></h1>
    Label: 
</section>
<section>
    <h1><a href=""></a></h1>
    Label: 
</section>
<section>
    <h1><a href=""></a></h1>
    Label: 
</section>
<section>
    <h1><a href=""></a></h1>
    Label: 
</section>

The documentation says:

…collections are also available under site.collections, with the metadata you specified in your _config.yml…

Is what I’m attempting correct? Or is this a bug?

Thanks—and congratulations on shipping v2!