RFR: 8373243 : EnumSet.spliterator() should specify and document its characteristics

[Viktor Klang]

Addresses https://bugs.openjdk.org/browse/JDK-8373243 by copying and adapting the specification from https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/util/LinkedHashSet.java#L186-L204

Since EnumSet is sealed and only permits two final classes, the verbiage around "Implementations should document the reporting of additional characteristic values." may be considered to get removed from this PR. Kept, for now, for symmetry reasons.

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Commit messages:
 - Adding Javadoc and specification for EnumSet::spliterator()

Changes: https://git.openjdk.org/jdk/pull/28696/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28696&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373243
  Stats: 19 lines in 1 file changed: 19 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/28696.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28696/head:pull/28696

PR: https://git.openjdk.org/jdk/pull/28696