RFR: 8310892: ScopedValue throwing StructureViolationException should be clearer

[Alan Bateman]

Docs only update to add a missing `@throws StructureViolationException` and make it clearer when the exception is thrown. In the future we might re-visit this so that the description is in one place rather than in each method.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/14679/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14679&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310892
  Stats: 60 lines in 1 file changed: 12 ins; 6 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/14679.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14679/head:pull/14679

PR: https://git.openjdk.org/jdk/pull/14679