RFR: 8338891: HotSpotDiagnosticsMXBean missing @since tag

[Kevin Walls]

Trivial "since" tag addition in javadoc: since 1.6

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Commit messages:
 - 8338891: HotSpotDiagnosticsMXBean missing @since tag

Changes: https://git.openjdk.org/jdk/pull/20823/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20823&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338891
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20823.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20823/head:pull/20823

PR: https://git.openjdk.org/jdk/pull/20823