RFR: 8321561: (fs) Add an API note to Files.move clarifying possible non-atomic behavior

[Brian Burkhalter]

Make it clear that a `FileAlreadyExistsException` may be thrown by `Files.move` even if the `REPLACE_EXISTING` option is specified.

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Commit messages:
 - 8321561: (fs) Add an API note to Files.move clarifying possible non-atomic behavior

Changes: https://git.openjdk.org/jdk/pull/17061/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17061&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321561
  Stats: 9 lines in 1 file changed: 9 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17061.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17061/head:pull/17061

PR: https://git.openjdk.org/jdk/pull/17061