'Third-party Signed' object does not say who the `signatures` are supposed to be from or why they are there

**Link to problem area**: https://spec.matrix.org/v1.9/client-server-api/#post_matrixclientv3joinroomidoralias

**Issue**
The *Third-party Signed* object, used for redeeming a 3PID invite, contains a `signatures` field.
But it doesn't say who the signatures are supposed to be *from*.

It also doesn't say why the signatures are needed, which can help with understanding.

Open the linked Client-Server API section for POST /_matrix/client/v3/join/{roomIdOrAlias} and read the Third-party Signed object definition. Update the specification to state who provides the signatures and why they are required, then confirm both points are clear in that section.

Written by the indexing model from the issue text.