RFR: JDK-8151531: Add notes to BaseStream.spliterator/iterator docs regarding them being escape hatches

[Viktor Klang]

Still relevant to address? /cc @PaulSandoz

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Commit messages:
 - JDK-8151531: Add notes to BaseStream.spliterator/iterator docs regarding them being escape hatches

Changes: https://git.openjdk.org/jdk/pull/13890/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13890&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8151531
  Stats: 10 lines in 1 file changed: 10 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/13890.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13890/head:pull/13890

PR: https://git.openjdk.org/jdk/pull/13890