Update CONTRIBUTING and templates to clarify process for Editorial and Ambiguity changes.

Ambiguity about process for spec ambiguity! Meta!

After recently re-reading our RFC stages criteria, I'm still confident in those definitions. However I've noticed that a lot of recent changes to the spec have not been RFCs, they've mostly been editorial and clarifying ambiguities. I think we should set clearer definitions for when a change will be considered editorial, when it will be considered an ambiguity clarification, and when it is an RFC. While we have a great process for RFCs, we lack those for the other two change types.