doc: clarify v5.1.1 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
* Soften language for CVE-2015-3193 as impact may not be as great as
  expected, also trim to match v4.2.3 language
* Clarify that CVE-2015-3194 affects TLS servers using _client
  certificate authentication_
* Include reference to CVE-2015-6764

PR-URL: #4156
Reviewed-By: Minwoo Jung <jmwsoft@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

CHANGELOG.md

@@ -4,12 +4,12 @@
 
 ### Notable changes
 
-* **http**: Fix a bug where an HTTP socket may no longer have a socket but a pipelined request triggers a pause or resume, a potential denial-of-service vector. (Fedor Indutny)
+* **http**: Fix CVE-2015-8027, a bug whereby an HTTP socket may no longer have a parser associated with it but a pipelined request attempts to trigger a pause or resume on the non-existent parser, a potential denial-of-service vulnerability. (Fedor Indutny)
 * **openssl**: Upgrade to 1.0.2e, containing fixes for:
-  - CVE-2015-3193 "BN_mod_exp may produce incorrect results on x86_64", an attack is considered feasible against DH, an attack against RSA and DSA is considered possible but unlikely, EC algorithms are not affected. Details are available at <http://openssl.org/news/secadv/20151203.txt>.
-  - CVE-2015-3194 "Certificate verify crash with missing PSS parameter", a potential denial-of-service vector for Node.js TLS servers; TLS clients are also impacted. Details are available at <http://openssl.org/news/secadv/20151203.txt>.
+  - CVE-2015-3193 "BN_mod_exp may produce incorrect results on x86_64", an attack may be possible against a Node.js TLS server using DHE key exchange. Details are available at <http://openssl.org/news/secadv/20151203.txt>.
+  - CVE-2015-3194 "Certificate verify crash with missing PSS parameter", a potential denial-of-service vector for Node.js TLS servers using client certificate authentication; TLS clients are also impacted. Details are available at <http://openssl.org/news/secadv/20151203.txt>.
   (Shigeki Ohtsu) [#4134](https://github.com/nodejs/node/pull/4134)
-* **v8**: Backport fixes for a bug in `JSON.stringify()` that can result in out-of-bounds reads for arrays. (Ben Noordhuis)
+* **v8**: Backport fix for CVE-2015-6764, a bug in `JSON.stringify()` that can result in out-of-bounds reads for arrays. (Ben Noordhuis)
 
 ### Known issues