[whatwg/url] Clearly mark conformance checking-only aspects of the IDNA section (Issue #816)

### What is the issue with the URL Standard?

When reading https://url.spec.whatwg.org/#idna in isolation to find out what kind of IDNA library surface the URL Standard needs, it's not apparent what parts of that section are used by User Agents that are not conformance checkers and which are extra parts for conformance checking but not by usual processing.

It would be useful to more clearly mark this in the IDNA section of the spec itself without the reader having to work this out by following the references backwards.

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