Mark internal slots that the spec always initializes.

Marking them instead of saying their initial value is `undefined`, makes
it easier to see if I accidentally introduce an uninitialized path.

index.bs

@@ -1654,14 +1654,14 @@ spec: permissions
       </thead>
       <tr>
         <td><dfn>\[[context]]</dfn></td>
-        <td>`undefined`</td>
+        <td>&lt;always initialized></td>
         <td>
           The {{Bluetooth}} object that returned this {{BluetoothDevice}}.
         </td>
       </tr>
       <tr>
         <td><dfn>\[[representedDevice]]</dfn></td>
-        <td>`undefined`</td>
+        <td>&lt;always initialized></td>
         <td>
           The <a>Bluetooth device</a> this object represents.
         </td>
@@ -1679,7 +1679,7 @@ spec: permissions
       </tr>
       <tr>
         <td><dfn>\[[allowedServices]]</dfn></td>
-        <td>`undefined`</td>
+        <td>&lt;always initialized></td>
         <td>
           This device's {{AllowedBluetoothDevice/allowedServices}} list for this origin
           or `"all"` if all services are allowed.