Docs: Explain HTTP header name munging -- please review before merging!

Docs: Explain HTTP header name munging in ssl_client_header and ssl_client_verify_header

The way these settings map to actual HTTP header names isn't transparent, so
this commit explains how they map.

I'd like this to be reviewed for accuracy by someone who knows the turf before merging. Specifically:

* That's how it works, right? That's how it looks like it works. 
* Is it Rack that's doing the munging, or some other layer?