message: fix annotation of parse_device_{added,removed}

gst_structure_id_get() returns a new reference so the returned device is
actually (transfer full).

The code using this API was already correct but the code example in
comments was not.

https://bugzilla.gnome.org/show_bug.cgi?id=768776

gst/gstdevicemonitor.c

@@ -49,12 +49,14 @@
  *          name = gst_device_get_display_name (device);
  *          g_print("Device added: %s\n", name);
  *          g_free (name);
+ *          gst_object_unref (device);
  *          break;
  *        case GST_MESSAGE_DEVICE_REMOVED:
  *          gst_message_parse_device_removed (message, &device);
  *          name = gst_device_get_display_name (device);
  *          g_print("Device removed: %s\n", name);
  *          g_free (name);
+ *          gst_object_unref (device);
  *          break;
  *        default:
  *          break;

gst/gstmessage.c

@@ -2383,7 +2383,7 @@ gst_message_new_device_added (GstObject * src, GstDevice * device)
 /**
  * gst_message_parse_device_added:
  * @message: a #GstMessage of type %GST_MESSAGE_DEVICE_ADDED
- * @device: (out) (allow-none) (transfer none): A location where to store a
+ * @device: (out) (allow-none) (transfer full): A location where to store a
  *  pointer to the new #GstDevice, or %NULL
  * 
  * Parses a device-added message. The device-added message is produced by
@@ -2435,7 +2435,7 @@ gst_message_new_device_removed (GstObject * src, GstDevice * device)
 /**
  * gst_message_parse_device_removed:
  * @message: a #GstMessage of type %GST_MESSAGE_DEVICE_REMOVED
- * @device: (out) (allow-none) (transfer none): A location where to store a
+ * @device: (out) (allow-none) (transfer full): A location where to store a
  *  pointer to the removed #GstDevice, or %NULL
  *
  * Parses a device-removed message. The device-removed message is produced by