Add 'onadvertisementreceived' attribute to BluetoothDeviceEventHandlers

This event type was defined but the EventHandler attribute was missing
from the BluetoothDevice interface. This change adds it to the
BluetoothDeviceEventHandlers mixin so that it is also present on the
Bluetooth interface for bubbled events.

Fixes #426.

index.bs

@@ -4980,9 +4980,15 @@ spec: promises-guide-1
       <pre class="idl">
         [SecureContext]
         interface mixin BluetoothDeviceEventHandlers {
+          attribute EventHandler onadvertisementreceived;
           attribute EventHandler ongattserverdisconnected;
         };
       </pre>
+      <p>
+        <dfn attribute for="BluetoothDeviceEventHandlers">onadvertisementreceived</dfn>
+        is an <a>Event handler IDL attribute</a> for the
+        {{advertisementreceived}} event type.
+      </p>
       <p>
         <dfn attribute for="BluetoothDeviceEventHandlers">ongattserverdisconnected</dfn>
         is an <a>Event handler IDL attribute</a>