RFR: 8322751: ZGC: Fix comments about marking roots

[Erik Österlund]

On Wed, 27 Dec 2023 08:54:56 GMT, Guoxiong Li <gli at openjdk.org> wrote:

> Hi all,
> 
> This trivial patch fixes comments about marking roots.
> 
> Thread-stack processing has been concurrent after [JDK-8253180](https://bugs.openjdk.org/browse/JDK-8253180).
> And the code about marking root in `ZMark::start` has been removed after [JDK-8254562](https://bugs.openjdk.org/browse/JDK-8254562).
> But some comments in ZGC and generational ZGC have not fixed, which may mislead the newbies of the ZGC.
> 
> Thanks for review.
> 
> Best Regards,
> -- Guoxiong

It looks trivial; ship it!

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PR Comment: https://git.openjdk.org/jdk/pull/17195#issuecomment-1873839132