Informative response also in CoRAL

[marco-tiloca-sics]

Migrated from https://gitlab.com/crimson84/draft-tiloca-core-observe-responses-multicast/-/issues/20

Original content and comments below

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Appendix A.1 gives a starting point

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In the appendix on alternative retrieval methods, we are now essentially having examples of (looking-alike) informative responses, both in CBOR and CoRAL. It's probably better to keep it this way than adding more (identical) examples inline in the document body.