Add method description to `PopupMenu`

This pull request adds a missing method description to `PopupMenu`.

This completes the documentation for `PopupMenu` and enhances usability by doing so.

Update doc/classes/PopupMenu.xml

Co-authored-by: Rémi Verschelde <[email protected]>

doc/classes/PopupMenu.xml

@@ -260,6 +260,7 @@
 			<return type="int">
 			</return>
 			<description>
+				Returns the index of the currently focused item. Returns [code]-1[/code] if no item is focused.
 			</description>
 		</method>
 		<method name="get_item_accelerator" qualifiers="const">