GIO: Add more missing GI annotations

- g_io_extension_point_register()
 - g_io_extension_point_lookup()
 - g_io_extension_point_implement()

https://bugzilla.gnome.org/show_bug.cgi?id=625383

gio/giomodule.c

@@ -643,8 +643,8 @@ g_io_extension_point_free (GIOExtensionPoint *ep)
  *
  * Registers an extension point.
  *
- * Returns: the new #GIOExtensionPoint. This object is owned by GIO
+ * Returns: (transfer none): the new #GIOExtensionPoint. This object is
- *    and should not be freed
+ *    owned by GIO and should not be freed.
  */
 GIOExtensionPoint *
 g_io_extension_point_register (const char *name)
@@ -681,8 +681,8 @@ g_io_extension_point_register (const char *name)
  *
  * Looks up an existing extension point.
  *
- * Returns: the #GIOExtensionPoint, or %NULL if there is no
+ * Returns: (transfer none): the #GIOExtensionPoint, or %NULL if there
- *    registered extension point with the given name
+ *    is no registered extension point with the given name.
  */
 GIOExtensionPoint *
 g_io_extension_point_lookup (const char *name)
@@ -826,7 +826,7 @@ extension_prio_compare (gconstpointer  a,
  * If @type has already been registered as an extension for this 
  * extension point, the existing #GIOExtension object is returned.
  *
- * Returns: a #GIOExtension object for #GType
+ * Returns: (transfer none): a #GIOExtension object for #GType
  */
 GIOExtension *
 g_io_extension_point_implement (const char *extension_point_name,