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