Clarify that the iteration order of `PersistentProperty` instances in `PersistentEntity` is not defined

We should clarify that both the iteration order for the `PersistentProperty` instances within a `PersistentEntity` and the `doWith…` methods is undefined to avoid folks relying on a particular order.

Related StackOverflow question [here](https://stackoverflow.com/questions/75922080/whats-the-order-of-persistentproperty-are-iterated-over-for-a-spring-data-persi).