Serialization javadoc updates to address handling of abstract value classes, and to clean up spec for Externalizable.
--------- - [x] I confirm that I make this contribution in accordance with the [OpenJDK Interim AI Policy](https://openjdk.org/legal/ai). ------------- Commit messages: - whitespace fixes - 8385170: [lworld] Serialization spec needs to allow abstract value classes like Number Changes: https://git.openjdk.org/valhalla/pull/2522/files Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=2522&range=00 Issue: https://bugs.openjdk.org/browse/JDK-8385170 Stats: 57 lines in 3 files changed: 29 ins; 2 del; 26 mod Patch: https://git.openjdk.org/valhalla/pull/2522.diff Fetch: git fetch https://git.openjdk.org/valhalla.git pull/2522/head:pull/2522 PR: https://git.openjdk.org/valhalla/pull/2522
