RFR: 8366970: CPUTimeUsage: Add comment about usage during VM exit

[Jonas Norlinder]

Some of the methods in CPUTimeUsage make a query on the underlying threads. Calling those methods during VM exit is therefore unsafe. This PR adds a comment to provide clarification.

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Commit messages:
 - Add comment

Changes: https://git.openjdk.org/jdk/pull/27115/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27115&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366970
  Stats: 3 lines in 1 file changed: 3 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/27115.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27115/head:pull/27115

PR: https://git.openjdk.org/jdk/pull/27115