RFR: 8322503: Shenandoah: Clarify gc state usage [v2]

[William Kemper]

> This is a follow up to https://github.com/openjdk/jdk/pull/17112.

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

  Add comments to gc state methods

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Changes:
  - all: https://git.openjdk.org/jdk/pull/17163/files
  - new: https://git.openjdk.org/jdk/pull/17163/files/13fe4565..49ad3452

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17163&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17163&range=00-01

  Stats: 7 lines in 1 file changed: 7 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17163.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17163/head:pull/17163

PR: https://git.openjdk.org/jdk/pull/17163