Update Preamble documentation for clusters with mixed request sources and loadbalancer chains

Summary:
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.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11487

Differential Revision: https://secure.phabricator.com/D16403