[1.6.x] Fixed #22013 -- Documented difference between SERVER_EMAIL and DEFAULT_FROM_EMAIL.
[1.6.x] Fixed #22013 -- Documented difference between SERVER_EMAIL and DEFAULT_FROM_EMAIL.

Backport of 44814e78cc from master

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