RFR: 8349344: Clarify documentation of Arena.ofConfined

[Per Minborg]

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.

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Commit messages:
 - Clarify docs for Arena.ofConfined()

Changes: https://git.openjdk.org/jdk/pull/23435/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23435&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8349344
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/23435.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23435/head:pull/23435

PR: https://git.openjdk.org/jdk/pull/23435