Update did:nft and did:safe specs to explain how to set Caip10Link, and the benefits of doing so

[stbrody]

CIP-94 (did:nft) and CIP-101 (did:safe) both have whole sections on how to read the Caip10Link from Ceramic to set the controller property of the resulting DID document, but doesn't have any information on how or when to set the Caip10Link. Nor does it explain the benefit of doing so, ie what the inclusion of the optional controller property to the DID document does.