Clarify "can also discover application groups by [...] /.well-known/core"

[chrysn]

Current draft says:

CoAP endpoints can also discover application groups by performing a group discovery query using the /.well-known/core resource. Such a request may be sent to a known CoAP group multicast address associated to application group(s), or to the All CoAP Nodes multicast address.

I don't quite understand how that'd happen; I see two possible interpretations:

• Use .wk/c to discover content of application groups, eg. by requesting coap://mygroup/.well-known/core, picking really only one response, and hoping that it's just the content of the group resources and not everything on the host (cf. Encoding the group name in requests #28 -- coap://mygroup/.well-known/core was requested, but what comes back is coap://[fe80::f00]/.well-known/core actually).
• Send out a broad multicast and wait for links that describe groups. However, other than the RD Appendix A methods (which aren't really applicable here), I don't know of any description format to be used. Is "The description of the group may be provided in an application specific or a yet to be defined generic format." meant here?

[EskoDijk]

Above PR expands the text with discovery use cases and examples. We may still expand this later on with discovery examples or the link format attribute(s) defined by draft-tiloca-core-oscore-discovery; but that is not done yet. (I would need some more time still to read that draft first)