Fixed #23822 -- Added support for serializing model managers in migration

Ticket: https://code.djangoproject.com/ticket/23822

- [x] Implementation
- [x] User documentation
- [x] Reference documentation
- [x] Tests
I don't think either wording is clear as to what it means (it means "managers with `use_in_migrations=True`"). Maybe best to just go for that longer but clearer wording?