Clarify that one cannot use the same variable

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