Fixed #20579 -- Improved TransactionTestCase.available_apps.

Also moved its documentation to the 'advanced' section. It doesn't
belong to the 'overview'. Same for TransactionTestCase.reset_sequences.

When available_apps is set, after a TransactionTestCase, the database
is now totally empty. post_syncdb is fired at the beginning of the next
TransactionTestCase.

Refs #20483.