Document implementation incompatibilities and specification ambiguities

[fauno]

I was talking with Bengo about documenting everything that we found confusing about the spec and implementation details so they can point to it on their work.

Maybe creating a label for issues?

[fauno]

• Many things depend on correct Content-Type headers in responses, assuming objects will be requested via HTTP or similar headers+body protocol. This leads to many endpoints missing a .json extension.

[fauno]

Mastodon (at least) expects the Create/Update activity to include the Note activity on the same object and won't dereference it if you send the object URL, so it leads to duplicate information. AFAICT the spec doesn't say anything regarding this, but it allows activities to be nested or referenced, so it was weird when Mastodon didn't dereference it.