Update TEL reference documentation

[jasoncolburne]

Rationale

The reference documentation has gone stale - I noticed a couple things during implementation/integration that could be corrected. I will also see if there is room for enhancement.

In particular:

• ri is required in rev events
• a nonce, n can be added to VC events so their saids are not easily guessable/constructable

[jasoncolburne]

I can't assign myself but I'm working on this