1. Externalizable classes are shown as just implementing Serializable, not Externalizable (and any other implemented interfaces)
    * at a minimum, if the class implements `Externalizable`, that should be shown instead of `Serializable`
    * the names could/should be linked in the standard way, if possible

2. The @param info for parameters of serializable methods is not displayed

3. The initial word should probably not always be`Class`. It should use `utils.getTypeElementKindName`

4. It's not clear that resource strings should be used to compose the heading. Maybe the code should just build up a `ContentBuilder` object, as in
    * type element kind name
    * class name
    * optional `extends superclass` ... that being said, only classes show up in this page, and the only class without a supertype is `Object` which is not `Serializable` so a there's always a supertype, unless we decide to suppress `extends Object`
    * `implements superinterfaces

4a. The heading is a weird mix of adjective (like `Class`) and keywords (like `extends` and `implements`)
It's not clear what the best strategy is for a heading

4b. Suggestion for the heading: two parts
    * main heading: <element-type-kind> <name> (e.g. Class foo)
    * subheading or inline: a "standard" "signature block", giving details of the class signature

5. when serializable classes are in a package, the enclosing package is used as a heading. The package name in the heading should probably link back to the main page for the package.

6. JDK-8249903 (fix in serialized form test) could be merged into this.