Updated migration docs to match changes from a159b1fa

Some things to note:

* You don't need to learn too much about ``ProjectState`` to just write simple

  migrations; just know that it has a ``.render()`` method that turns it into

  migrations; just know that it has an ``apps`` property that gives access to

  an app registry (which you can then call ``get_model`` on).

* ``database_forwards`` and ``database_backwards`` both get two states passed