RFR: 8338780: GenShen: Fix up some comments

[Y. Srinivas Ramakrishna]

In https://github.com/openjdk/jdk/pull/20395 Aleksey provided some suggestions for improving various comments. This ticket gathers together several of those changes.

**Testing:**
- [x] GHA
- [x] SPECjbb w/GenShen
- [x] jtreg:tier1
- [ ] Codepipeline testing

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Commit messages:
 - Merge branch 'master' into review_changes_2
 - JDK-8338780 GenShen: Fix up some comments

Changes: https://git.openjdk.org/shenandoah/pull/484/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=484&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338780
  Stats: 87 lines in 3 files changed: 35 ins; 45 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/484.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/484/head:pull/484

PR: https://git.openjdk.org/shenandoah/pull/484