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

Thanks CollinAnderson for the report.

Backport of 7de35a9ef3 from master

ccn-2m/django,hasadna/django,ccn-2m/django,imtapps/django-imt-fork,bliti/django-nonrel-1.5,bliti/django-nonrel-1.5,hasadna/django,imtapps/django-imt-fork,imtapps/django-imt-fork,hasadna/django,ccn-2m/django,bliti/django-nonrel-1.5,ccn-2m/django