[jdk17u-dev] RFR: 8284874: Add comment to ProcessHandle/OnExitTest to describe zombie problem

[Goetz Lindenmaier]

I am backporting this to simplify follow-up backports. Comment only, harmless.

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Commit messages:
 - Backport bb7c97bddfe88cb3261706f5e272fd0418e5238c

Changes: https://git.openjdk.org/jdk17u-dev/pull/2906/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2906&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284874
  Stats: 19 lines in 1 file changed: 18 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2906.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2906/head:pull/2906

PR: https://git.openjdk.org/jdk17u-dev/pull/2906