[1.5.x] Fixed #19588 - Added create_superuser to UserManager docs.
[1.5.x] Fixed #19588 - Added create_superuser to UserManager docs.

Thanks minddust for the report.

Backport of 4da5947a87 from master

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