<i18n dev> RFR: 8357275: Locale.Builder.setLanguageTag should explicitly state extlangs are allowed

[Justin Lu]

It is not clear that `Locale.Builder.setLanguageTag(String)` accepts _extlang_ subtags in the input as well as what behavior occurs. Additionally, both this method and `Locale.forLanguageTag(String)` should mention their behavior when more than three _extlang_ subtags are provided. This PR clarifies the lack of context in the specification.

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Commit messages:
 - init

Changes: https://git.openjdk.org/jdk/pull/25309/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25309&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8357275
  Stats: 7 lines in 1 file changed: 4 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/25309.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25309/head:pull/25309

PR: https://git.openjdk.org/jdk/pull/25309