AD review: clarify extensibility of "intuse" claim

[gmandyam]

See https://github.com/ietf-rats-wg/eat/issues/379

[laurencelundblade]

Agree that it should be possible to define new values.

I don't know if we can get away with saying that a registry can be created in the future, but am fine trying that.

Another thing possible here might be to say values above 64K are proprietary and below are reserved for a future IANA registry.

Agree with the wording suggestions.

[carl-wallace]

There's a draft in LAMPS that has language on establishing a registry in the future (draft-ietf-lamps-header-protection).