Update Signed Doc spec

[no30bit]

Summary

There's minor inconsistencies in Signed doc spec. Eliminating them improves determinism.

Description

• collaborators field (discussion) – change from "optional zero or more" to "optional one or more". Currently, an empty array and no field mean the same thing, which isn't deterministic.
• CoseSign payload (discussion) – specify that payload can't be detached in Signed Doc case. Thus, forbid nil payload.
• section field (discussion) – is it a json pointer or a json path? The spec and the implementation are inconsistent, so fix either one or both.