Improve docs on how to load/save FrozenNodeIterator

docs/module/nodeiterator.rst

@@ -25,6 +25,13 @@ Iterator :class:`NodeIterator` and a context manager
 .. autoclass:: FrozenNodeIterator
    :no-show-inheritance:
 
+   A serializable representation of a :class:`NodeIterator` instance, saving
+   its iteration state.
+
+   It can be serialized and deserialized with :func:`save_structure_to_file`
+   and :func:`load_structure_from_file`, as well as with :mod:`json` and
+   :mod:`pickle` thanks to being a :func:`~collections.namedtuple`.
+
 ``resumable_iteration``
 """""""""""""""""""""""