RFR: Incorporate feedback from reviewers [v2]

[William Kemper]

> This PR incorporates changes made in response to https://github.com/openjdk/jdk/pull/14185

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Add comment explaining that plabs are aligned to card sizes

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/285/files
  - new: https://git.openjdk.org/shenandoah/pull/285/files/7348eda4..2edc2e01

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=285&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=285&range=00-01

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/shenandoah/pull/285.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/285/head:pull/285

PR: https://git.openjdk.org/shenandoah/pull/285