Add a note to the doc for g_viarnat_get_child_value() that the return
value is non-floating and must be freed with g_variant_unref().
https://bugzilla.gnome.org/show_bug.cgi?id=654412
* It is an error if @index_ is greater than the number of child items
* in the container. See g_variant_n_children().
*
+ * The returned value is never floating. You should free it with
+ * g_variant_unref() when you're done with it.
+ *
* This function is O(1).
*
* Since: 2.24