RFR: 8306785: fix deficient spliterators for Sequenced Collections

[Paul Sandoz]

On Wed, 12 Jul 2023 17:25:03 GMT, Stuart Marks <smarks at openjdk.org> wrote:

> Change some reverse-ordered spliterators to use `Spliterators.spliterator(Collection, ORDERED)` instead of `Spliterators.spliteratorUnknownSize()`. This gives SIZED and ORDERED spliterators. More optimizations will be done later. I'm intending to backport this to JDK 21 so I want this to be minimal.
> 
> I've also included some adjustments to spliterator implementation comments to clear up a few points of confusion. (No spec changes.)

This looks like a reasonable low risk change, plus good clarifications on comments.

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Marked as reviewed by psandoz (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14854#pullrequestreview-1527372524