RFR: 8351077: Shenandoah: Update comments in ShenandoahConcurrentGC::op_reset_after_collect

[Xiaolong Peng]

This is a trivial PR to update the code comments in ShenandoahConcurrentGC::op_reset_after_collect.

After doing more test and analysis, we have a better understanding why reset bitmap of young gen after concurrent cycle may cause crash if there is pending old GC cycle to execute: When there is soft reference in old gen, but the referent is in young, reseting bitmap of young will cause wrong state of the soft reference, which may lead to expected cashes.

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Commit messages:
 - 8351077: Shenandoah: Update comments in ShenandoahConcurrentGC::op_reset_after_collect

Changes: https://git.openjdk.org/jdk/pull/23872/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23872&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351077
  Stats: 5 lines in 1 file changed: 0 ins; 2 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23872.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23872/head:pull/23872

PR: https://git.openjdk.org/jdk/pull/23872