Restore the origin-bound declaration on DigitalCredential

Reverts #509. DigitalCredential is origin bound. Restoring the declaration lets
Credential Management's opaque-origin check, which rejects a request for an
origin-bound credential from an opaque origin, apply to DigitalCredential. See the
companion Credential Management PR.

Author: marcoscaceres

index.html

@@ -1513,6 +1513,9 @@ <h3>
       static boolean userAgentAllowsProtocol(DOMString protocol);
     };
     </pre>
+    <p>
+      {{DigitalCredential}} instances are [=Credential/origin bound=].
+    </p>
     <h4>
       The `protocol` member
     </h4>