Relax requirement to use HTTP 501

[m-mohr]

The API currently says:

404 Not Found: The resource specified by the path does not exist, i.e. one of the resources belonging to the specified identifiers are not available at the back-end. Note: Unsupported endpoints MUST use HTTP status code 501.

This is pretty wild as the default for unsupported endpoints in many environments is 404. Thus you'd need to implement special handling to specifically all endpoints you don't want to implement (in the future). None of the clients (AFAIK) and also none of the back-ends really makes use of the 501, so we could also just weaken the requirement and say:

Note: Unsupported endpoints MAY use HTTP status code 501.