(SERVER-104) Added docs for external SSL termination

This may be too esoteric to explain here, but I think it would be worth noting that the certificate in a "X-Client-Cert"  header will only be used for operations pertaining to the application-level Ruby Puppet code, e.g., enforcing access per auth.conf rules and for trusted node information.  During the low-level SSL handshake between the client and the webserver, the certificate that the webserver validates against its CA will be the one provided over the SSL/TLS protocol.  @cprice404, think we should include something about this or not?