RFR: 8309853: StructuredTaskScope.join description improvements

[Alan Bateman]

StructuredTaskScope's class description introduces the join method as waiting for all subtasks to finish but the API docs for join/joinUntil are phrased in terms of waiting for all threads to finish. ShutdownOnXXX join/joinUntil inherit this description but would be clearer if their description said they wait until all subtasks finished or a subtask to succeed or fail.

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Commit messages:
 - Add apiNote about using shutdown directly
 - Merge
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14419/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14419&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309853
  Stats: 82 lines in 1 file changed: 54 ins; 3 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/14419.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14419/head:pull/14419

PR: https://git.openjdk.org/jdk/pull/14419