RFR: 8367237: Thread-Safety Usage Warning for java.text.Collator Classes

[Justin Lu]

Please review this PR which is a documentation change to make apparent the recommended approach for using Collator & RuleBasedCollator in a multi threaded environment via an implNote. The original issue and CSR have additional context.

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Commit messages:
 - Reword w/o thread-safe
 - Reword
 - init

Changes: https://git.openjdk.org/jdk/pull/27233/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27233&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367237
  Stats: 16 lines in 2 files changed: 9 ins; 4 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27233.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27233/head:pull/27233

PR: https://git.openjdk.org/jdk/pull/27233