[webauthn] Pull Request: Update signature counters section.

agl has just submitted a new pull request for https://github.com/w3c/webauthn:

== Update signature counters section. ==
This section did not reflect the specified behaviour for signature
counters and did not mention that they are returned in makeCredential
responses too. See linked bug for details.

Fixes #1370

See https://github.com/w3c/webauthn/pull/1390