RFR: 8349344: Clarify documentation of Arena.ofConfined

[Jorn Vernee]

On Tue, 4 Feb 2025 09:46:48 GMT, Per Minborg <pminborg at openjdk.org> wrote:

> This PR proposes to clarify the documentation for `Arena.ofConfined()`. It is proposed to say that segments allocated from the returned `Arena` can _only_ be accessed by the thread that created the `Arena` in the first place.

Marked as reviewed by jvernee (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/23435#pullrequestreview-2593736433