[gtk+] docs: add docs for GtkTreeSortableIface

[William Jon McCann <mccann src gnome org>]

commit a64ffa2958ed747238128120a0297246ac708477
Author: William Jon McCann <william jon mccann gmail com>
Date:   Mon Jan 20 23:13:27 2014 -0500

    docs: add docs for GtkTreeSortableIface

 gtk/gtktreesortable.h |   18 ++++++++++++++++++
 1 files changed, 18 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtktreesortable.h b/gtk/gtktreesortable.h
index 4c46845..ae3cabe 100644
--- a/gtk/gtktreesortable.h
+++ b/gtk/gtktreesortable.h
@@ -71,10 +71,28 @@ typedef gint (* GtkTreeIterCompareFunc) (GtkTreeModel *model,
                                         gpointer      user_data);
 
 
+/**
+ * GtkTreeSortableIface:
+ * @sort_column_changed: Signal emitted when the sort column or sort
+ *    order of sortable is changed.
+ * @get_sort_column_id: Fills in sort_column_id and order with the
+ *    current sort column and the order.
+ * @set_sort_column_id: Sets the current sort column to be
+ *    sort_column_id.
+ * @set_sort_func: Sets the comparison function used when sorting to
+ *    be sort_func.
+ * @set_default_sort_func: Sets the default comparison function used
+ *    when sorting to be sort_func.
+ * @has_default_sort_func: %TRUE if the model has a default sort
+ * function.
+ */
 struct _GtkTreeSortableIface
 {
+  /*< private >*/
   GTypeInterface g_iface;
 
+  /*< public >*/
+
   /* signals */
   void     (* sort_column_changed)   (GtkTreeSortable        *sortable);