RFR: 8330178: Clean up non-standard use of /** comments in `java.base`

[Jonathan Gibbons]

Please review a set of updates to clean up use of `/**` comments in the vicinity of declarations.

There are various categories of update:

* "Box comments" beginning with `/**`
* Misplaced doc comments before package or import statements
* Misplaced doc comments after the annotations for a declaration
* Dangling `/**` comments relating to a group of declarations, separate from the doc comments for each of those declarations
* Use of `/**` for the legal header at or near the top of the file

In one case, two doc comments before a declaration were merged, which fixes a bug/omission in the documented serialized form.

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Commit messages:
 - JDK-8330178: Clean up non-standard use of /** comments in `java.base`

Changes: https://git.openjdk.org/jdk/pull/18846/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18846&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8330178
  Stats: 134 lines in 23 files changed: 50 ins; 56 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/18846.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18846/head:pull/18846

PR: https://git.openjdk.org/jdk/pull/18846