RFR: 8327790: Improve javadoc for ResolvedJavaType.hasFinalizableSubclass

[Doug Simon]

This PR fixes and clarifies the javadoc for `ResolvedJavaType.hasFinalizableSubclass()`.

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Commit messages:
 - fix javadoc for return type of ResolvedJavaType.hasFinalizableSubclass

Changes: https://git.openjdk.org/jdk/pull/18192/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18192&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327790
  Stats: 5 lines in 1 file changed: 2 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/18192.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18192/head:pull/18192

PR: https://git.openjdk.org/jdk/pull/18192