gen-art-33

- "The AP sends the response…" -> "The AP MUST send the response…".
- "…can directly connect…" -> "MAY directly connect".
- "it can still populate" -> "it MAY still populate".
- Reworded the Ping description with normative language.
- "JP simply sends" -> "JP MUST send".

Closes #164
Merge pull request #164 from petithug/gen-art-33

Gen-ART 33