doc: clarify v0.12.9 notable items

* Include reference to CVE-2015-8027
* Fix "socket may no longer have a socket" reference
* Expand on non-existent parser causing the error
* Clarify that CVE-2015-3194 affects TLS servers using _client
  certificate authentication_

PR-URL: https://github.com/nodejs/node/pull/4154


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>