<i18n dev> RFR: JDK-8327875: ChoiceFormat should advise throwing UnsupportedOperationException for unused methods

[Justin Lu]

Please review this PR which advises ChoiceFormat subclasses throw `UnsupportedOperationException` for unused inherited methods.

The CSR covers the proposed wording, and the reasons as to why this is a specification and not an implementation update. In addition, the `api/implNote` tags have been moved to the bottom of the class description, as they are no longer only relevant to the pattern section, (where they were previously located).

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Commit messages:
 - remove contract wording
 - move and clarify api/impl note
 - init

Changes: https://git.openjdk.org/jdk/pull/18499/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18499&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327875
  Stats: 25 lines in 1 file changed: 15 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18499.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18499/head:pull/18499

PR: https://git.openjdk.org/jdk/pull/18499