Discuss extensibility of extension points through type or group sockets

[rmarx]

With #415 and the upcoming PR describing general extensibility, we won't yet have discussed guidelines for extending other stuff (e.g., $QuicFrame or $H3Parameters or $H3FrameType), so don't forget to add those once those two PRs are merged.

At that point, probably also best to move the socket extensibility discussion to the general extensibility section and add this new thing there as well.

(this is mostly a DO NOT FORGET ABOUT THIS ROBIN issue)