docs: clarify guarantees of the File Chooser API

The application should not depend on where the URL points, or how
access to the file is granted, just that access has been granted at
that URL.

data/org.freedesktop.portal.FileChooser.xml

@@ -28,8 +28,7 @@
       backend will present the user with a file chooser dialog.
 
       The selected files will be made accessible to the application
-      via the document portal, and the returned URI will point
-      into the document portal fuse filesystem in ``/run/user/$UID/doc/``.
+      (which may involve adding it to the document portal).
 
       This documentation describes version 4 of this interface.
   -->