[1.6.x] Fixed #22507 -- Clarified nature of the sender argument of signals
[1.6.x] Fixed #22507 -- Clarified nature of the sender argument of signals

Backport of d1f93e9c1e from master.

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