doc: update for TLS 1.3

This updates the documentation regarding TLS 1.3, according to the blog article. After going through the existing documentation, I realized that almost all points in #443 (closed) (quoted below) have been already covered.

• upgrade section
  • include information from blog post
• main body
  • ensure major new features are mentioned
    • re-key + auth re-key
    • post-handshake-auth