RFR: 8276186: Require getAvailableLocales() methods to include Locale.ROOT [v2]

[Naoto Sato]

> This fix is to require to include `Locale.ROOT` in the returned arrays/set from `getAvailableLocales()` methods in various locale-sensitive classes. The implementation has been including `Locale.ROOT` since its inception, it is simply a doc clarification (+ a test case verifying it). Corresponding CSR has also been drafted: https://bugs.openjdk.java.net/browse/JDK-8276249

Naoto Sato has updated the pull request incrementally with one additional commit since the last revision:

  Using @code tag for `Set`.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/6258/files
  - new: https://git.openjdk.java.net/jdk/pull/6258/files/1a611bd5..5bdd4917

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=6258&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=6258&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6258.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6258/head:pull/6258

PR: https://git.openjdk.java.net/jdk/pull/6258