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