"no comment" warnings for non-public entities related to serialization

[Florian Weimer]

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.