Allow unhiding credentials.

index.bs

@@ -3121,6 +3121,9 @@ follows:
     |credential| from the [=credentials map=] or employ an
     [=authenticator=]-specific procedure to hide it from future [=authentication
     ceremonies=].
+1. Else, if |credential| has been hidden by an [=authenticator=]-specific
+    procecure, reverse the action so that |credential| is present in future
+    [=authentication ceremonies=].
 
 <div class="example">
 
@@ -3153,7 +3156,10 @@ e.g. on every sign in.
 Note: Credentials not present in
 {{AllAcceptedCredentialIdsOptions/allAcceptedCredentialIds}} will be removed or
 hidden, potentially irreversibly. [=Relying parties=] must exercise care that
-valid credential IDs are never omitted from the list.
+valid credential IDs are never omitted from the list. If a valid [=credential
+ID=] were accidentally omitted, [=relying parties=] should immediately include
+them in {{PublicKeyCredential/signalAllAcceptedCredentialIds(options)}} as soon
+as possible to "unhide" them, if supported by the [=authenticator=].
 
 #### {{PublicKeyCredential/signalCurrentUserDetails(options)}} #### {#sctn-signalCurrentUserDetails}