RFR: 8350178: Incorrect comment after JDK-8345580

[SendaoYan]

Hi all,
In [JDK-8345580](https://bugs.openjdk.org/browse/JDK-8345580), the const modifier for variable Node::_idx has been removed, but the constant description for variable Node::_idx leave unchange. The related description should be updated.

Only touch the comments, no risk.

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Commit messages:
 - 8350178: Incorrect comment after JDK-8345580

Changes: https://git.openjdk.org/jdk/pull/23659/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23659&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350178
  Stats: 4 lines in 1 file changed: 0 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/23659.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23659/head:pull/23659

PR: https://git.openjdk.org/jdk/pull/23659