Add explicit documentation and regression tests for the use of django.db.models.F()-expressions when assigning directly to attributes on model instances (as opposed to just using them in batch update() calls).
The pattern goes something like this:
>>> from django.db.models import F
>>> from myapp.models import MyModel
>>> obj = MyModel(count=1)
>>> obj.save()
>>> obj.count
1
>>> obj.count = F('count') + 2
>>> obj.save()
>>> obj = MyModel.objects.get(pk=obj.pk) # Necessary to reload here, as the `count` attribute will still be an Expression object.
>>> obj.count
3
This should not work on records which do not already exist, since F()-expressions only work in UPDATE queries.
It's also important for this to work with any pre-save and post-save signal handlers defined in any of the django.contrib apps.
