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

[William Kemper]

On Wed, 20 Dec 2023 23:54:00 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> 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

Added comments to new methods and synced up with tip to see if it addresses GHA build failure on macosx.

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PR Comment: https://git.openjdk.org/jdk/pull/17163#issuecomment-1865307929