Updated the documentation for savepoints.

a database for which the behavior applies.  If no ``using`` argument is

provided then the ``"default"`` database is used.

Savepoints are controlled by three methods on the transaction object:

Savepoints are controlled by three functions in :mod:`django.db.transaction`:

.. method:: transaction.savepoint(using=None)

.. function:: savepoint(using=None)

    Creates a new savepoint. This marks a point in the transaction that

    is known to be in a "good" state.

    Returns the savepoint ID (sid).

    Returns the savepoint ID (``sid``).

.. method:: transaction.savepoint_commit(sid, using=None)

.. function:: savepoint_commit(sid, using=None)

    Updates the savepoint to include any operations that have been performed

    since the savepoint was created, or since the last commit.

    Releases savepoint ``sid``. The changes performed since the savepoint was

    created become part of the transaction.

.. method:: transaction.savepoint_rollback(sid, using=None)

.. function:: savepoint_rollback(sid, using=None)

    Rolls the transaction back to the last point at which the savepoint was

    committed.

    Rolls back the transaction to savepoint ``sid``.

These functions do nothing if savepoints aren't supported or if the database

is in autocommit mode.

In addition, there's a utility function:

.. function:: clean_savepoints(using=None)

    Resets the counter used to generate unique savepoint IDs.

The following example demonstrates the use of savepoints::