<i18n dev> RFR: 8347121: Add missing @serial tags to module java.base

[Hannes Wallnöfer]

Please review a doc-only change to add missing `@serial` javadoc tags in module `java.base`. This is a sub-task of [JDK-8286931] to allow us to re-enable the javadoc `-serialwarn` option in the JDK doc build, which has been disabled since JDK 19.

[JDK-8286931]: https://bugs.openjdk.org/browse/JDK-8286931

For private and package-private serialized fields that already have a doc comment, the main description is converted to a block tag by prepending `@serial` since these fields do not require a main description. (For protected and public serialized fields we would have appended an empty `@serial` block tag instead with the same effect, but there are no protected/public fields in this particular sub-task.) For those fields that do not have a doc comment, a doc comment with an empty `@serial` tag is added. 

There is one special case in `java/util/UUID.java` where two private fields had plain traditional comments containing `@serial` tags instead of doc comments. Converting these comments to doc-comments is the only change in this PR that causes a change in the generated documentation, adding documentation for these two fields to the `serialized-form.html` page.

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Commit messages:
 - 8347121: Add missing @serial tags to module java.base

Changes: https://git.openjdk.org/jdk/pull/22978/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22978&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347121
  Stats: 305 lines in 89 files changed: 83 ins; 4 del; 218 mod
  Patch: https://git.openjdk.org/jdk/pull/22978.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22978/head:pull/22978

PR: https://git.openjdk.org/jdk/pull/22978