Made changes to the invalid placeholder handling in
`ListFormat.getInstance(String[])`, stemming from
[JDK-8385736](https://bugs.openjdk.org/browse/JDK-8385736).
This rejects duplicate placeholders and `"{2}"` outside the `three` pattern,
with spec and test updates. Also includes minor Javadoc formatting cleanups.
The invalid-long-pattern test added with
[JDK-8385736](https://bugs.openjdk.org/browse/JDK-8385736) has been repurposed
to cover the newly specified invalid-placeholder cases, because the
repeated-placeholder input is now rejected immediately by duplicate-placeholder
validation.
Since this changes the behavior of the method, a corresponding CSR has been
drafted.
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Interim AI Policy](https://openjdk.org/legal/ai).
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Commit messages:
- initial commit
Changes: https://git.openjdk.org/jdk/pull/31377/files
Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=31377&range=00
Issue: https://bugs.openjdk.org/browse/JDK-8385834
Stats: 65 lines in 2 files changed: 34 ins; 1 del; 30 mod
Patch: https://git.openjdk.org/jdk/pull/31377.diff
Fetch: git fetch https://git.openjdk.org/jdk.git pull/31377/head:pull/31377
PR: https://git.openjdk.org/jdk/pull/31377
