RFR: JDK-8300042: Improve CPU related JFR events descriptions

[Matthias Baesken]

There are a number of JFR events dealing with CPU related information. Those sometimes miss descriptions or have very limited descriptions, this can be improved.

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Commit messages:
 - JDK-8300042

Changes: https://git.openjdk.org/jdk/pull/11969/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11969&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8300042
  Stats: 9 lines in 1 file changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11969.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11969/head:pull/11969

PR: https://git.openjdk.org/jdk/pull/11969