RFR: 8309821: Link to hidden classes section in Class specification for Class::isHidden

[Joe Darcy]

Simple doc improvement.

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Commit messages:
 - JDK-8309821: Link to hidden classes section in Class specification for Class::isHidden

Changes: https://git.openjdk.org/jdk/pull/19781/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19781&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309821
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19781.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19781/head:pull/19781

PR: https://git.openjdk.org/jdk/pull/19781