Update Preamble documentation for clusters with mixed request sources and loadbalancer chains
Fixes T11487. Improve documentation for three situations:
- When you configure a cluster behind a load balancer, all requests are trusted but not all have an "X-Forwarded-For" header. Change the suggested snippet to read this header only if it exists.
- When a request goes through a series of load balancers (as with a CDN) they can end up writing a list of IPs to the header. Parse these.
- Remove the "rate limiting" stuff -- this got disabled/removed a long time ago and is misleading/incorrect.
Test Plan: Read documentation.
Reviewed By: chad
Maniphest Tasks: T11487
Differential Revision: https://secure.phabricator.com/D16403