Fixed #20687 -- Added documentation for django.core.signing API.

Thanks Baptiste Mispelon for the suggestion.
Fixed #20609 -- Documented how to use request.user with RequestFactory

Thanks michel@ for the suggestion.
Fixed #20675 -- `check_password` should work when no password is specified.

The regression was introduced by 2c4fe761a. refs #20593.
Fixed #20690 -- Mentioned LOCALE_PATHS earlier in translation docs.

Thanks rene@ for the suggestion.
Fixed #12579 -- Noted QuerySet.get_or_create() depends on database unique constraints.

Thanks timmolendijk, jdunck, vijay_shanker, and loic84.
Fixed #20673 -- Clarified that HttpRequest.user uses AUTH_USER_MODEL.

Thanks littlepig for the report.
Fixed #20134 -- Correct list of fields that UserManager requires.

Thanks semenov and pegler.