[1.6.x] Fixed #21710 -- Documented User.get_short_name()
[1.6.x] Fixed #21710 -- Documented User.get_short_name()

Thanks Keryn Knight for the report.

Backport of 0150830687 from master

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