ACTION-364: cert-related terminology

I've merged Stephen's changes.  I haven't yet done a full pass
through the document (which is about due); one open point is that,
currently, we seem to have lost the "what is a secure page"
definition.

I suspect that that implies a follow-up action item for either
Stephen or myself.

Regards,
-- 
Thomas Roessler, W3C  <tlr@w3.org>