Write message.md

A skeleton message.md that ties the header/body/footer currently exists, but needs more work.

Acceptance criteria:
- Get alignment on message Overview wording
- Ensure Structure is complete and correct in description
- Security Considerations considered and written

Start by reading message.md and the existing header, body, and footer sections. Review the Overview wording, complete and correct the Structure description, and write the Security Considerations section; done means all three acceptance criteria are addressed in message.md.

Written by the indexing model from the issue text.