Should we specify that the `alsoKnownAs` MUST include prior values for the DID after a move?

This was raised as part of [this issue](https://github.com/decentralized-identity/didwebvh-info/issues/32). Do we, and should we, require that an `alsoKnownAs` be in the DIDDocs after a DID move? To this point, the only requirement we put on the DIDDoc be that it contains the DID as the value of root `id` attribute -- that is necessary to bind the DID to the SCID from inception. Do we want to add an additional requirement after a move?