(SERVER-108) Minor touchups for the external SSL termination docs

This commit includes the following changes:

* Modified the language in the `external_ssl_termination.markdown` doc
  to hopefully help clarify how/when a client certificate from SSL vs.
  in an `X-Client-Cert` header is used.

* Added a link to the `external_ssl_termination.markdown` doc from the
  `README.md` doc.

* Removed the note about `SERVER-18` from `known_issues.markdown` since
  this issue has now been addressed.