mention that serialize_container should be deterministic

arraycontext/container/__init__.py

@@ -122,7 +122,10 @@ def serialize_container(ary: ArrayContainer) -> Iterable[Tuple[Any, Any]]:
     r"""Serialize the array container into an iterable over its components.
 
     The order of the components and their identifiers are entirely under
-    the control of the container class.
+    the control of the container class. However, the order is required to be
+    deterministic, i.e. two calls to :func:`serialize_container` on the same
+    array container should return an iterable with the components in the same
+    order.
 
     If *ary* is mutable, the serialization function is not required to ensure
     that the serialization result reflects the array state at the time of the