ACTION-348: cert related terminology

Hi all,

Please find attached a proposed rewrite of section 5.

Main changes suggested are:

- tweaked terminology & loads of editorial changes
- elided terms:
	- no interaction cert: since there's no spec. at which to point
	- attested cert: couldn't see the need (am open to correction)

I guess this ought be put on an agenda sometime before being put
back into the doc.

Regards,
Stephen.