j.u.c.a *Adder and *Accumulator extend a package private class that is Serializable

This bug is for part 2 of JDK-8026220 whose description is below:

A serialized stream includes the ObjectStreamClass of the superclass if the superclass is Serializable. So I'm starting to conclude that we have two issues:

1. A javadoc bug. Taking StringBuilder as an example then its superclass is the package-private AbstractStringBuilder (not Serializable) and there is no reference to this in the serialized stream. So I think javadoc needs to filter this out, as it does in the class view.

2. A j.u.c. problem because *Adder and *Accumulator extend a package private class that is Serializable (because Striped64 extends Number). As a check, I've serialized a DoubleAccumulator instance and the bytes include a reference to Striped64.