[gtk+] Clarify that one cannot use the same variable

[Matthias Clasen <matthiasc src gnome org>]

commit 28fc4de6aaed944d223c7a84ecb6ef3c6d340e3b
Author: Wouter Verhelst <w uter be>
Date:   Thu Mar 31 11:09:41 2016 +0200

    Clarify that one cannot use the same variable
    
    A naive way to perform an action on all parent nodes of a given node
    could be to do:
    
    while(gtk_tree_model_iter_parent(model, &iter, &iter)) {
        /* perform some action on iter here */
    }
    
    However, since gtk_tree_model_iter_parent() will initialize the iterator
    pointed to by the second parameter before performing the lookup, this
    will not work.
    
    Explicitly document this behaviour.
    
    Signed-off-by: Wouter Verhelst <w uter be>
    
    https://bugzilla.gnome.org/show_bug.cgi?id=573380

 gtk/gtktreemodel.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/gtk/gtktreemodel.c b/gtk/gtktreemodel.c
index 5e447b9..500658b 100644
--- a/gtk/gtktreemodel.c
+++ b/gtk/gtktreemodel.c
@@ -1640,6 +1640,9 @@ gtk_tree_model_iter_nth_child (GtkTreeModel *tree_model,
  * @child will remain a valid node after this function has been
  * called.
  *
+ * @iter will be initialized before the lookup is performed, so @child
+ * and @iter cannot point to the same memory location.
+ *
  * Returns: %TRUE, if @iter is set to the parent of @child
  */
 gboolean