RFR: 8220816: (fs) Files.createDirectory should make it more obvious that it fails when the directory already exists

[Brian Burkhalter]

Slightly changes the specification of `Files.createDirectory` to highlight that it fails if a filesystem entity already exists at the given location.

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Commit messages:
 - 8220816: (fs) Files.createDirectory should make it more obvious that it fails when the directory already exists

Changes: https://git.openjdk.org/jdk/pull/28378/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28378&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8220816
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28378.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28378/head:pull/28378

PR: https://git.openjdk.org/jdk/pull/28378