[webauthn] Merged Pull Request: Add note about extensions not using ext ID as entry key

equalsJeffH has just merged emlun's pull request 1439 for https://github.com/w3c/webauthn:

== Add note about extensions not using ext ID as entry key ==
Also updates the example to illustrate this and remove irrelevant details.

Fixes #1430.


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<a href="https://pr-preview.s3.amazonaws.com/w3c/webauthn/pull/1439.html" title="Last updated on Jun 10, 2020, 10:12 PM UTC (76c93dd)">Preview</a> | <a href="https://pr-preview.s3.amazonaws.com/w3c/webauthn/1439/2824aa4...76c93dd.html" title="Last updated on Jun 10, 2020, 10:12 PM UTC (76c93dd)">Diff</a>

See https://github.com/w3c/webauthn/pull/1439