Add a note about the /status/ path for load balancers to setup docs

Summary: Fixes T12926. This exists but isn't documented. Document it after the section about webserver setup, since that's probably when you'd want to set it up.

Test Plan: Read carefully, visited /status/.

Reviewers: chad, amckinley

Reviewed By: chad

Maniphest Tasks: T12926

Differential Revision: https://secure.phabricator.com/D18234