RFR: 8377179: Improve and document racy use of start/end in ThreadLocalAllocBuffer

[Thomas Schatzl]

Hi all,

  please review this change that tries to document and streamline the racy use of `ThreadLocalAllocBuffer` internals.

Testing: gha

Thanks,
  Thomas

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Commit messages:
 - * more documentation
 - * more improvements
 - * improve documentation
 - * copyright update
 - 8377179

Changes: https://git.openjdk.org/jdk/pull/29573/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29573&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377179
  Stats: 61 lines in 3 files changed: 13 ins; 25 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/29573.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29573/head:pull/29573

PR: https://git.openjdk.org/jdk/pull/29573