Update xAPI.md

5.1-5.3 - note State language removed from 5.1 to go generic across documents.  Requirement was "added" as it existed within the table.
5.3 saw non-requirement "shoulds" go into details, the rest separated into requirements.  Requirements around IRI "owners" and the "permission" were dropped in favor of language just saying that they should be controlled.  This "requirement" may be a bit much, but didn't want to risk moving to a non-patch version.