Tests and docs complement to cecc0791.

Fixed #28298 -- Prevented a primary key alteration from adding a foreign key constraint if db_constraint=False.

Thanks Tim Graham and Markus Holtermann for review.

MigrationRecorder now assumes that if the django_migrations table
doesn't exist, then no migrations are applied.

Reverted documentation change from refs #23808.

In retrospect, implementing a __repr__() for index subclasses doesn't
seem worthwhile.

Fixed #27539 -- Made TransactionTestCase._pre_setup() clear the queries_log so it's less likely to overflow.

TransactionTestCase.assertNumQueries() fails in an overflow situation.

Thanks Tim Graham for the review and edits.

Mistake in 97c1931c.