docs: Improve GtkTreeModel iteration pattern

Iterating over the model in this way means that use of continue is less
error-prone, as the increment is part of the loop construct.

https://bugzilla.gnome.org/show_bug.cgi?id=548793

gtk/gtktreemodel.c

@@ -168,12 +168,12 @@
  * /* Fill the list store with data */
  * populate_model (list_store);
  *
- * /* Get the first iter in the list */
- * valid = gtk_tree_model_get_iter_first (list_store, &iter);
- *
- * while (valid)
+ * /* Get the first iter in the list, check it is valid and walk
+ *  * through the list, reading each row. */
+ * for (valid = gtk_tree_model_get_iter_first (list_store, &iter);
+ *      valid;
+ *      valid = gtk_tree_model_iter_next (list_store, &iter))
  *  {
- *    /* Walk through the list, reading each row */
  *    gchar *str_data;
  *    gint   int_data;
  *
@@ -190,7 +190,6 @@
  *    g_free (str_data);
  *
  *    row_count++;
- *    valid = gtk_tree_model_iter_next (list_store, &iter);
  *  }
  * </programlisting>
  * </example>