RFR: 8322751: ZGC: Fix comments about marking roots

[Guoxiong Li]

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

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Commit messages:
 - 8322751: ZGC: Fix comments about marking roots

Changes: https://git.openjdk.org/jdk/pull/17195/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17195&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322751
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/17195.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17195/head:pull/17195

PR: https://git.openjdk.org/jdk/pull/17195