Fixed #24645 -- Documented use of HStoreExtension.

    A field for storing mappings of strings to strings. The Python data type

    used is a ``dict``.

    To use this field, you'll need to setup the hstore extension in PostgreSQL

    by adding a migration with the

    :class:`~django.contrib.postgres.operations.HStoreExtension` operation.

    You'll see an error like ``'type "hstore" does not exist'`` if you skip

    this step.

.. note::

    On occasions it may be useful to require or restrict the keys which are