Document termination of children on SIGHUP
Document termination of children on SIGHUP

Change I3e7264a1efcbf66a9afc69d8ed20f600c985c296 chenged behavior of
services when they receive SIGHUP. This needs to be documented.

Change-Id: I5a8de19ba4d7a5e2b27d88da8668f6171b7b2112

openstack/oslo.service