Any documentation about the elementindex template available?
Your best bet on learning element type templating is to look at Craft template itself. Element types use the following template for controlling the layout:

craft/app/templates/_layouts/elementindex.html

However, its important to know that you should only be using the elementindexif you are creating a custom element type.

As far as the element index, a good portion of the options are actually set through Javascript by Craft using Garnish. If you look at the following code, you can see that the status, template side, is set via Garnish. 

{% block initJs %}
Craft.createElementIndex('{{ elementTypeClass }}', $('#main'), {
    context:        '{{ context }}',
    showStatusMenu: {{ showStatusMenu is defined ? showStatusMenu|json_encode|raw : "'auto'" }},
    showLocaleMenu: {{ showLocaleMenu is defined ? showLocaleMenu|json_encode|raw : "'auto'" }},
    storageKey:     'elementindex.{{ elementTypeClass }}',
    criteria:       { localeEnabled: null }
});
{% endblock %}


You can set custom statuses on your element type, if that is the route you are looking to travel. I have dug into element types and custom statuses on these posts:


How to integrate custom statuses in a workflow style custom element type?
Best way to use custom enums in my plugin templates?
Best way to implement my own statuses on my custom element type?


As for Craft documentation, they are working on it still and the only thing available right now is this: 

Creating new Element Types