Adds requirement language for activity id to be consistent with those next to agent and registration
Switch language to be "used by" rather than generated to match that under "registration". "Used by" seems to be more consistent because the values only need to be "generated" once, as they need to be the same on subsequent launches so are only generated once.
Switch "Registration ID" to just "registration" and lower case to be more consistent with how "actor" is represented (this follows a change from "Registration ID" to "registration" in a PR that was accepted in the WG mtg on 05/07).
Resolves #634