room directory does not make it clear how to find a resident server

**Link to problem area**:

https://spec.matrix.org/v1.5/server-server-api/#get_matrixfederationv1publicrooms
https://spec.matrix.org/v1.5/client-server-api/#get_matrixclientv3publicrooms

**Issue**

Neither the C-S nor federation `/publicRooms` APIs make it clear if servers may return rooms that they are no longer resident in (or cannot handle a join request for some other reason).

If servers *can* return rooms that they are no longer resident in, how is the client supposed to join the room, given there is no server list?

Start with the linked Matrix v1.5 federation and client-server /publicRooms sections. Trace whether a returned room must be resident and how a client can join when no server list is provided; done means the specification states the permitted behavior and join path unambiguously.

Written by the indexing model from the issue text.