Fixed #23597 -- Clarified {Single,Multiple}ObjectMixin.model docs

https://code.djangoproject.com/ticket/23597

I added a new "Default managers" heading under the Managers topic to be able to refer to the paragraph I wanted to. (I also think it's probably important enough to warrant its own heading).
I believe a right apostrophe is required here, otherwise sphinx will render it in the opposite direction. see c33d1ca1d98003de29cdecb6080b52c5c52139bd.