Rewrite the documentation for selecting diff revisions.

Our nice new revision selector UI is hopefully pretty self explanatory, but all
the same, we should document it. The nice thing about this new UI is that it's
much easier to explain how to choose an interdiff.

Testing done: Generated HTML docs.
Reviewed at http://reviews.reviewboard.org/r/4783/
Make use of the new web API optimizations in Djblets.

This helps the new optimizations for the Djblets web API code. It calls
the new _get_queryset, and makes use of the new id_field in get_object.