Undocumented --skip-systemctl-redirect option

I wondered why --skip-systemctl-redirect wasn't documented in the man_page for pg_ctlcluster?

I discovered it trying to figure out why running pg_ctlcluster from Saltstack on Ubuntu 16.04 was hanging - adding that
flagresolved the issue.  One of the first places I checked when running into the issue was the pg_ctlcluster man page,
soI think it would be a good idea to add there... 

Best wishes,
--
Casey