Add required mediation (#149)

Author: marcoscaceres

index.html

@@ -244,6 +244,10 @@ <h2>
       The <dfn>DigitalCredential</dfn> interface represents a conceptual
       [=digital credential=].
     </p>
+    <p>
+    [=User mediation=] is always {{CredentialMediationRequirement/"required"}}.
+    [=Request a credential|Requesting a DigitalCredential credential=] does not support {{CredentialMediationRequirement/"conditional"}}, {{CredentialMediationRequirement/"optional"}}, or {{CredentialMediationRequirement/"silent"}} [=user mediation=]. If {{CredentialsContainer/get()}} is called with anything other than {{CredentialMediationRequirement/"required"}}, a {{TypeError}} will be thrown.
+    </p>
     <pre class="idl">
     [Exposed=Window, SecureContext]
     interface DigitalCredential : Credential {