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.
Merge branch 'nfagerlund-header_munging_description'

* nfagerlund-header_munging_description:
  Docs: Explain HTTP header name munging in ssl_client_header and ssl_client_verify_header

[ci skip]
closes #1597