Improve documentation of exceptions (see ticket #20786)

For https://code.djangoproject.com/ticket/20786.

I've added information for importing each exception where it's listed (I think this is clearer than having it at the top of each section - often you arrive at these directly to the relevant exception, so it's easier to have the information available right by the exception, rather than at the top).

I've also fixed a typo, and clarified the docs for `DoesNotExist` and `ObjectDoesNotExist` (the documentation currently references `django.core.exceptions.DoesNotExist`, which does not exist).