RFR: JDK-8133773: clarify specification of Spliterator.tryAdvance

[Viktor Klang]

Attempting to make the "at-most once" nature, of invoking the supplied action to tryAdvance, clearer in the JavaDoc.

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Commit messages:
 - JDK-8133773: clarify specification of Spliterator.tryAdvance

Changes: https://git.openjdk.org/jdk/pull/13860/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13860&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8133773
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13860.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13860/head:pull/13860

PR: https://git.openjdk.org/jdk/pull/13860