[webauthn] Merged Pull Request: revise RP ID definition and Note

equalsJeffH has just merged equalsJeffH's pull request 970 for https://github.com/w3c/webauthn:

== revise RP ID definition and Note ==
fixes #963 
thx @engedy and @arnar for suggestions.


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See https://github.com/w3c/webauthn/pull/970