Fixed #6915: Documented the fact that QuerySet.delete() may not call delete()...

    Entry.objects.filter(pub_date__year=2005).delete()

Keep in mind that this will, whenever possible, be executed purely in

SQL, and so the ``delete()`` methods of individual object instances

will not necessarily be called during the process. If you've provided

a custom ``delete()`` method on a model class and want to ensure that

it is called, you will need to "manually" delete instances of that

model (e.g., by iterating over a ``QuerySet`` and calling ``delete()``

on each object individually) rather than using the bulk ``delete()``

method of a ``QuerySet``.

When Django deletes an object, it emulates the behavior of the SQL

constraint ``ON DELETE CASCADE`` -- in other words, any objects which

had foreign keys pointing at the object to be deleted will be deleted