RFR: 8262876: Shenandoah: Fix comments regarding VM_ShenandoahOperation inheritances

[Zhengyu Gu]

please review this comment only fix.

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Commit messages:
 - JDK-8262876

Changes: https://git.openjdk.java.net/jdk/pull/2796/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2796&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8262876
  Stats: 7 lines in 1 file changed: 4 ins; 3 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2796.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2796/head:pull/2796

PR: https://git.openjdk.java.net/jdk/pull/2796