Mention some cautions in section overriding delete()

[FunkyELF]

I strongly feel that this section...

​http://docs.djangoproject.com/en/dev/topics/db/models/#overriding-predefined-model-methods

... should have a note or something like the following.

Note that the delete() method for an object is not necessarily called when deleting a queryset containing that object as explained in ​http://docs.djangoproject.com/en/dev/topics/db/queries/#deleting-objects.
To ensure a customized delete method gets called have a look at ​http://docs.djangoproject.com/en/dev/topics/signals/#topics-signals
Also note that when deleting an object from the built in admin interface related objects may be deleted in this way and may not call the overridden delete() method.

[Russell Keith-Magee]

A note to this effect already exists in the queryset.delete() documentation (which you can get to if you follow the links from the "overriding model methods" page), but I agree it's worth repeating -- or at least drawing attention to the existing docs.

[Tim Graham]

(In [15071]) Fixed #12313 - Add a note that QuerySet.delete() doesn't necessarily call obj.delete(). thanks FunkyELF for the suggestion.

[Tim Graham]

(In [15072]) [1.2.X] Fixed #12313 - Add a note that QuerySet.delete() doesn't necessarily call obj.delete(). thanks FunkyELF for the suggestion.

Backport of r15071 from trunk.