Define preventSilentAccess() behavior (#758)

Merged per the decision at the Monterey plenary meeting

index.bs

@@ -1371,6 +1371,18 @@ When this method is invoked, the user agent MUST execute the following algorithm
 
 </div>
 
+###  Preventing silent access to an existing credential - PublicKeyCredential's `[[preventSilentAccess]](credential, sameOriginWithAncestors)` method ### {#preventSilentAccessCredential}
+
+<div link-for-hint="PublicKeyCredential/[[preventSilentAccess]](credential, sameOriginWithAncestors)">
+
+Calling the <dfn for="PublicKeyCredential" method>\[[preventSilentAccess]](credential, sameOriginWithAncestors)</dfn> method
+will have no effect on authenticators that require an [=authorization gesture=],
+but setting that flag may potentially exclude authenticators that can operate without user intervention.
+
+This [=internal method=] accepts no arguments.
+
+</div>
+
 ### Availability of User-Verifying Platform Authenticator - PublicKeyCredential's `isUserVerifyingPlatformAuthenticatorAvailable()` method ### {#isUserVerifyingPlatformAuthenticatorAvailable}
 
 <div link-for-hint="WebAuthentication/isUserVerifyingPlatformAuthenticatorAvailable">