RFR: 8359919: Minor java.util.concurrent doc improvements

[Doug Lea]

This collects miscellaneous open issues that can be resolved with documentation updates; each indicated by adding JDK issue numbers

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Commit messages:
 - Revert
 - Merge remote-tracking branch 'refs/remotes/origin/master' into JDK-8359919
 - Initial set of doc updates
 - Merge branch 'openjdk:master' into JDK-8355026
 - Reduce branchiness of Exchanger

Changes: https://git.openjdk.org/jdk/pull/25880/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8359919
  Stats: 164 lines in 7 files changed: 15 ins; 1 del; 148 mod
  Patch: https://git.openjdk.org/jdk/pull/25880.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25880/head:pull/25880

PR: https://git.openjdk.org/jdk/pull/25880