* clutter/clutter-model-default.c:
(clutter_model_default_iter_next),
(clutter_model_default_iter_prev): Do not instantiate a new
iterator, and update the passed one instead, as we say we do
in the documentation. This avoid leaking tons of iterators.
* clutter/clutter-model.[ch]: Update the documentation.
* tests/test-model.c: Prettify some output.