Document a pre-existing but little known capability of the {% comment %} tag

The {% comment %} tag has a nice feature, and has since the start, but it's little known.
This patch documents the ability to add a meta note to a comment:
```
{% comment 'disabled for performance reasons, see bug #121' %}
    code...
{% endcomment %}
```
Which is super handy, and works in all past versions of django.  There's a matching trac ticket for this.