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
Try to make `versionadded::` blocks "self-contained" so we can delete the entire thing when we remove them instead of having to deindent text and reflow it. For example, this text would appear above and the annotation would simply be something like "The `managers` parameter was added."