improve <rateLimiter /> doc

[rrayst]

Most setups will have one trusted gateway in front of Membrane.

Therefore the use case RateLimitInterceptorTest.getClientIpTrustXForwardedFor() should be highlighted (and described) in the Javadoc as well as in the examples/rateLimiter/proxies.xml.

The Javadoc could also mention (as the comment in the getClientIpTrustXForwardedFor() does) that by the time the interceptor is called, Membrane will (in most setups) have added the remote IP to X-Forwarded-For.