[1.6.x] Clarified why one must not catch database errors inside atomic.
[1.6.x] Clarified why one must not catch database errors inside atomic.

Backport of 4db2752 from master.

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