I'm not sure how recent this change is, but I noticed that "no comment" warnings are now printed for entities that are related to serialization: non-public classes which implement Serializable, and private non-transient fields in serializable classes, and named private serialization-related methods.
I think it might make sense to add a few words to the warning that it's about serialization compatibility. It took me a moment to realize why javadoc was warning about these constructs.
