Fixed #24880 -- Added more explicit docs on select_for_update() on SQLite.

:exc:`~django.db.DatabaseError` to be raised. This is in order to prevent code

unexpectedly blocking.

Evaluating a queryset with ``select_for_update()`` in autocommit mode is

a :exc:`~django.db.transaction.TransactionManagementError` error because the

Evaluating a queryset with ``select_for_update()`` in autocommit mode on

backends which support ``SELECT ... FOR UPDATE`` is a

:exc:`~django.db.transaction.TransactionManagementError` error because the

rows are not locked in that case. If allowed, this would facilitate data

corruption and could easily be caused by calling code that expects to be run in

a transaction outside of one.

Using ``select_for_update()`` on backends which do not support

``SELECT ... FOR UPDATE`` (such as SQLite) will have no effect.

``SELECT ... FOR UPDATE`` will not be added to the query, and an error isn't

raised if ``select_for_update()`` is used in autocommit mode.

.. warning::