[JEP 390] Revise "value-based class" & apply to wrappers

Update ValueBased.html to reflect the JEP's constraints on inline class migration:

 - Is `final`,

  - Declares only `final` instance fields,

  - Extends `Object`, or a hierarchy of abstract classes that declare no instance
    fields and have no instance initialization logic,

  - Declares only `private` constructors, or has deprecated its constructors for
    removal,

  - Does not promise a unique instance will be created with each factory method
    invocation (or any other instance creation mechanism),

  - Does not rely upon or expose object identity through any of its methods,

  - Overrides `toString`, `equals`, and `hashCode`, and

  - Declares no `synchronized` methods, and discourages clients from performing
    synchronization.

Update the specs of the 8 primitive wrapper classes (Boolean, Character, Byte, Short, Integer, Long, Float, Double) to indicate that they are value-based classes.

No longer designate subclasses of ConstantDesc as value-based, because most of these classes declare their own fields.