Fixed #17131 - Added per object permission notes to docs.

* Access to delete an object is limited to users with the "delete"

  permission for that type of object.

Permissions are set globally per type of object, not per specific object

instance. For example, it's possible to say "Mary may change news stories," but

it's not currently possible to say "Mary may change news stories, but only the

ones she created herself" or "Mary may only change news stories that have a

certain status, publication date or ID." The latter functionality is something

Django developers are currently discussing.

Permissions can be set not only per type of object, but also per specific

object instance. By using the

:meth:`~django.contrib.admin.ModelAdmin.has_add_permission`,

:meth:`~django.contrib.admin.ModelAdmin.has_change_permission` and

:meth:`~django.contrib.admin.ModelAdmin.has_delete_permission` methods provided

by the :class:`~django.contrib.admin.ModelAdmin` class, it is possible to

customize permissions for different object instances of the same type.

Default permissions

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