gnome: (VideoManager) document "file://" MRL

Refs #45480

Change-Id: I6d1791bc3bfd701532843b87cb467cc3fd9f4562

gnome/src/dbus/videomanager-introspec.xml

@@ -118,6 +118,7 @@
                     <ul>
                         <li>v4l2://DEVICE</li>
                         <li>display://DISPLAY_NAME[ WIDTHxHEIGHT]</li>
+                        <li>file://IMAGE_PATH</li>
                     </ul>
                 </tp:docstring>
             </arg>