It is unclear when `third_party_signed` property of `/join` endpoints should be used

**Link to problem area**:

- [`POST /_matrix/client/v3/join/{roomIdOrAlias}`](https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3joinroomidoralias)
- [`POST /_matrix/client/v3/rooms/{roomId}/join`](https://spec.matrix.org/v1.13/client-server-api/#post_matrixclientv3roomsroomidjoin)

**Issue**

It is unclear how or when to used this field. Reading the [Third-party invites](https://spec.matrix.org/unstable/client-server-api/#third-party-invites) section and the 3 involved APIs, it is not clear how a client would end up with this data, because it is not supposed to be involved in the process beyond the initial request to `/invite`.

A wild guess I would make is that it is possible to do that by using an out-of-bounds method, i.e. without a Matrix identity server. Then the client of the inviting user sends the `m.room.third_party_invite` manually and the client of the invited user uses `/join` with `third_party_signed`?

Related #1722.

Start with the two linked POST /join endpoints and the Third-party invites section, then compare how `third_party_signed` is described across those APIs. Review related issue #1722 and the existing two-comment discussion. Done means the specification clearly explains when a client receives and sends this field, or identifies the missing protocol behavior.

Written by the indexing model from the issue text.