Fixed #19881 -- Documented that get_next/previous_by_FOO uses default manager.

returns the next and previous object with respect to the date field, raising

a :exc:`~django.core.exceptions.DoesNotExist` exception when appropriate.

Both methods accept optional keyword arguments, which should be in the format

described in :ref:`Field lookups <field-lookups>`.

Both of these methods will perform their queries using the default

manager for the model. If you need to emulate filtering used by a

custom manager, or want to perform one-off custom filtering, both

methods also accept optional keyword arguments, which should be in the

format described in :ref:`Field lookups <field-lookups>`.

Note that in the case of identical date values, these methods will use the

primary key as a tie-breaker. This guarantees that no records are skipped or