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

[CIP-94](https://github.com/ceramicnetwork/CIP/blob/main/CIPs/CIP-94/CIP-94.md) (did:nft) and [CIP-101](https://github.com/ceramicnetwork/CIP/blob/main/CIPs/CIP-101/CIP-101.md) (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.