hspow: Clarifications to scheme and descriptor Item
- Fix inconsistent terminology
- Clarify handling of unknown schemes
- Declare that a future pow-params can have an object
IDK whether C Tor currently permits an Object in an unknown pow-params. IMO if it doesn't, that is wrong; it seems logically sensible to permit an Object here.
Noticed these ambiguities reviewing arti!2026 (closed), which answers Q3 the other way. I will make a review comment there.