doc: Add D-Bus global Capabilities property

This property was added to the fi.w1.wpa_supplicant1 interface in commit
1634ac0654eba8d458640a115efc0a6cde3bac4d ('dbus: Add global capabilities
property'), but documentation was not updated.

Signed-off-by: Jouni Malinen <j@w1.fi>

doc/dbus.doxygen

@@ -124,6 +124,11 @@ registered in the bus with fi.w1.wpa_supplicant1 name.
 	<h3>EapMethods - as - (read)</h3>
 	<p>An array with supported EAP methods names.</p>
       </li>
+
+      <li>
+	<h3>Capabilities - as - (read)</h3>
+	<p>An array with supported capabilities (e.g., "ap", "ibss-rsn", "p2p", "interworking").</p>
+      </li>
     </ul>
 
 \subsection dbus_main_signals Signals