R1:a35b03ac6ad9

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…