Tighten up documentation on SSL and HTTPS

[dsyer]

The section on HTTPS is actually mostly about how to live behind an HTTPS proxy. We could tighten up the wording a bit.

[tekul]

Also, I think RemoteIpValve, X-Forwarded-For etc may be useful behind any proxy which supports it, even if it doesn't have HTTPS enabled.

[dsyer]

Good point, thanks.