RFR: 8344853: Parallel: Improve comments in psParallelCompact

[Albert Mingkun Yang]

Trivial revising some existing comments and adding some comments. No real code change other than a renaming.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/22318/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22318&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344853
  Stats: 17 lines in 1 file changed: 1 ins; 1 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/22318.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22318/head:pull/22318

PR: https://git.openjdk.org/jdk/pull/22318