Fixed #20511 -- Corrected link about isolation levels in databases docs

.. versionadded:: 1.6

Like PostgreSQL itself, Django defaults to the ``READ COMMITTED`` `isolation

level <postgresql-isolation-levels>`_. If you need a higher isolation level

such as ``REPEATABLE READ`` or ``SERIALIZABLE``, set it in the

:setting:`OPTIONS` part of your database configuration in

:setting:`DATABASES`::

level`_. If you need a higher isolation level such as ``REPEATABLE READ`` or

``SERIALIZABLE``, set it in the :setting:`OPTIONS` part of your database

configuration in :setting:`DATABASES`::

    import psycopg2.extensions

    handle exceptions raised on serialization failures. This option is

    designed for advanced uses.

.. _postgresql-isolation-levels: http://www.postgresql.org/docs/current/static/transaction-iso.html

.. _isolation level: http://www.postgresql.org/docs/current/static/transaction-iso.html

Indexes for ``varchar`` and ``text`` columns

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