RFR: 8322503: Shenandoah: Clarify gc state usage [v3]

[William Kemper]

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

William Kemper has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Merge remote-tracking branch 'openjdk/master' into sync-gc-state-followup
 - Add comments to gc state methods
 - Rename gc state methods for clarity, assert that only java threads use thread local copy of gc state

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/17163/files
  - new: https://git.openjdk.org/jdk/pull/17163/files/49ad3452..db146b84

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=17163&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=17163&range=01-02

  Stats: 298 lines in 19 files changed: 209 ins; 53 del; 36 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