Fixes the guide for Pages ACL
What does this MR do?
The current version of the guide Access control when running GitLab Pages on a separate server implemented in gitlab-pages#211 (closed) does not work correctly. Opening Pages will fail with the error like:
{"host":"root.<ip-address>","level":"error","msg":"Authentication is not configured","path":"/pages-simple-test/","state":"","time":"2019-12-05T09:58:03Z"}
It turned out that it is also necessary to enable access control on a pages server to make it work. I have verified it on a test server and with one of our customers in https://gitlab.zendesk.com/agent/tickets/140269 (internal).
This MR adds the required option to the docs.