Synchronize DID role terminology between Use Cases and Core specs

[selfissued]

Section 2 of the Use Cases (What can you do with a DID?) defines three entities for working with DIDs: The Controller, the Relying Party, and the Subject. This seems highly related to w3c/did-core#4 - which is about defining terms for three DID roles.

The terms for these roles should be kept in sync between the specifications as they evolve.

[philarcher]

As discussed by the WG 2019-11-12 there will be a single terms document that will be included automatically in both the UCR and the Core spec (and maybe other docs). The FPWD, however, will use a snapshot that highlights this future state and the current open issues.