Add a note to the federation invite endpoints that invites can be sent twice

[anoadragon453]

As this may be non-obvious when implementing behaviour that is triggered by an incoming invite event.

See #2062 for more context. Closes #2062.

Preview: https://pr2067--matrix-spec-previews.netlify.app

[zecakeh]

Regarding the discussion in #2062, I actually saw this endpoint only as a request from the inviting homeserver to the invited homeserver to sign the event, not that it should have other side effects. But it's true that the description is vague and doesn't actually say what should happen on the invitee's side.

As a side note, it is also not clear in which order the signatures are applied here. Should the event already have the signature from the inviting homeserver? It would make sense that the invited homeserver checks the signature before signing it itself. The text doesn't say anything about it, and the definitions don't include a signatures field in the request (or the response for that matter), but the data has an example of a request with the signatures field (which appears in #2076).

[richvdh]

Should the event already have the signature from the inviting homeserver?

Synapse's implementation of PUT /_matrix/federation/v2/invite/{roomId}/{eventId} seems to require a valid signature on the incoming event, so I think the answer to this is "yes". That doesn't seem to be specced though, and it could do with being.

[richvdh]

I actually saw this endpoint only as a request from the inviting homeserver to the invited homeserver to sign the event, not that it should have other side effects.

It has to have a side-effect, at least in the case where the invited homeserver is not already in the room: the server must tell the user they have been invited and allow them to join the room if they wish.

[zecakeh]

I actually saw this endpoint only as a request from the inviting homeserver to the invited homeserver to sign the event, not that it should have other side effects.

It has to have a side-effect, at least in the case where the invited homeserver is not already in the room: the server must tell the user they have been invited and allow them to join the room if they wish.

Ah right, sorry for the noise then.

[richvdh]

@anoadragon453 could you make the requested changes please