[jdk17] RFR: 8269513: Clarify the spec wrt `useOldISOCodes` system property

[Naoto Sato]

Please review this small doc change to the system property. Accompanying CSR has also been created.

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Commit messages:
 - 8269513: Clarify the spec wrt `useOldISOCodes` system property

Changes: https://git.openjdk.java.net/jdk17/pull/163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk17&pr=163&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269513
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk17/pull/163.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk17 pull/163/head:pull/163

PR: https://git.openjdk.java.net/jdk17/pull/163