docs: Move GIO_USE_VFS to "okay for production" section

Lots of projects like NetworkManager, ostree, udisks, soon polkit,
etc.  do this or *should* do this.  And we need to support that
forever.

https://bugzilla.gnome.org/show_bug.cgi?id=767172

docs/reference/gio/overview.xml

@@ -361,11 +361,6 @@ Gvfs is also heavily distributed and relies on a session bus to be present.
       </para>
     </formalpara>
 
-    <para>
-      The following environment variables are only useful for debugging
-      GIO itself or modules that it loads. They should not be set in a
-      production environment.
-    </para>
     <formalpara>
       <title><envar>GIO_USE_VFS</envar></title>
 
@@ -374,10 +369,17 @@ Gvfs is also heavily distributed and relies on a session bus to be present.
         implementation to override the default for debugging purposes.
         The #GVfs implementation for local files that is included in GIO
         has the name "local", the implementation in the gvfs module has
-        the name "gvfs".
+        the name "gvfs".  Most commonly, system software will set this to "local"
+	to avoid having `GFile` APIs perform unnecessary DBus calls.
       </para>
     </formalpara>
 
+    <para>
+      The following environment variables are only useful for debugging
+      GIO itself or modules that it loads. They should not be set in a
+      production environment.
+    </para>
+
     <formalpara>
       <title><envar>GIO_USE_FILE_MONITOR</envar></title>