Advertising CIDs on Path ID != needs to use MP_NEW_CONNECTION_ID by design

[qdeconinck]

Coming from #334, there is currently a SHOULD that is not necessary. We should update the text to state that MP_NEW_CONNECTION_ID frames are used to allocate CIDs on a specific Path ID, and that NEW_CONNECTION_ID frames are equivalent to MP_NEW_CONNECTION_ID frames with Path ID 0. See original discussion below.

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          I don't understand the motivation behind

After a successful handshake indicating multipath support by both endpoints, endpoints SHOULD also use the MP_NEW_CONNECTION_ID and MP_RETIRE_CONNECTION_ID frames to provide new connection IDs or, respectively, retire them for the initial path with Path ID 0.

We can just say that MP_NEW_CONNECTION_ID with path ID 0 is equivalent to NEW_CONNECTION_ID. That's how we dealt with the fact that there's one bespoke value in RESET_STREAM_AT. It really doesn't matter which frame the peer uses, but in my code, I'd always go for the shortest encoding.

_Originally posted by @marten-seemann in https://github.com/quicwg/multipath/pull/334#discussion_r1606214314_

[mirjak]

-08 does not use normative language here anymore, so I'll close this issue. However, there is another related issue that can be used for discussion if needed which is #320