RFR: 8358618: UnsupportedOperationException no-args constructor javadoc is not clear

[Nizar Benalla]

Please review this patch that extends the javadoc of `UnsupportedOperationException` no-arg constructor, to clear up that the detail message is null.

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Commit messages:
 - improve UOE javadoc

Changes: https://git.openjdk.org/jdk/pull/26533/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26533&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8358618
  Stats: 6 lines in 2 files changed: 2 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/26533.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26533/head:pull/26533

PR: https://git.openjdk.org/jdk/pull/26533