Permissions documentation not up to date

[David Chandek-Stark]

​https://docs.djangoproject.com/en/dev/topics/auth/#permissions says:

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.

However, the ModelAdmin class provides has_add_permission, has_change_permission and has_delete_permission methods.

[mateusgondim]

I replaced the outdated text in auth.txt with a text talking about the possibility of permissions per specific object instance.

[Tim Graham <timograham@…>]

In [8d3e501502c308cbdd3cc95b62ace0fe11d373ab]:

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

Thanks dchandek for the suggestion and mateusgondim for the patch.

[Tim Graham <timograham@…>]

In [fa6577f5b2af9848b661fc24719b59e07d214728]:

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

Thanks dchandek for the suggestion and mateusgondim for the patch.

Backport of 8d3e501502c308cbdd3cc95b62ace0fe11d373ab from master