[1.6.x] Fixed #20779 -- Documented AdminSite.app_index_template; refs #8498.
[1.6.x] Fixed #20779 -- Documented AdminSite.app_index_template; refs #8498.

Thanks CollinAnderson for the report.

Backport of 7de35a9ef3 from master

felixjimenez/django,felixjimenez/django,felixjimenez/django,redhat-openstack/django,redhat-openstack/django,felixjimenez/django,django-nonrel/django,dex4er/django,django-nonrel/django,redhat-openstack/django,dex4er/django,dex4er/django,redhat-openstack/django,django-nonrel/django,django-nonrel/django