Update text on CoAP discovery, based on draft-join-proxy

[EskoDijk]

• CoAP text now mostly refers to draft-ietf-constrained-join-proxy which defines it
• GRASP part is left in (maybe still useful since we do the IANA notes on GRASP here)
• general text improvements proposed, clarifications.
• removed text about "unsecured wired link layers" which wasn't really needed I think (and hard to understand)
• removed the concept of "0 hops away" because there's always one hop even when Pledge/Registrar are link-local. We are talking here about hops between Pledge and Registrar, not hops between Join Proxy and Registrar. (The latter can be 0)
• removed section 11.5 that gave some wrong information (pointing to EST server discovery) while it should be just equal to Join Proxy discovery as defined in Section 10.