RFR: 8287749: Re-enable javadoc -serialwarn option

[Erik Joelsson]

On Wed, 26 Feb 2025 14:06:43 GMT, Hannes Wallnöfer <hannesw at openjdk.org> wrote:

> Please review a simple build change to re-enable the `-serialwarn` javadoc option for normal and reference API docs. The option was removed in [JDK-8252717](https://bugs.openjdk.org/browse/JDK-8252717) to avoid warnings for missing `@serial` tags which have since been added in [JDK-8286931](https://bugs.openjdk.org/browse/JDK-8286931). 
> 
> I tested normal and reference docs builds, both run without warning and the generated documentation is unchanged. The `@serial` tag in the Exception template is required and also does not affect javadoc output.

Marked as reviewed by erikj (Reviewer).

-------------

PR Review: https://git.openjdk.org/jdk/pull/23802#pullrequestreview-2644723376