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

[swcurran]

This was raised as part of this issue. 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?