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

src/java.base/share/classes/java/nio/file/Files.java

@@ -611,7 +611,8 @@ public static Path createFile(Path path, FileAttribute<?>... attrs)
     }
 
     /**
-     * Creates a new directory. The check for the existence of the file and the
+     * Creates a new directory, failing if the file or directory already exists.
+     * The check for the existence of the file and the
      * creation of the directory if it does not exist are a single operation
      * that is atomic with respect to all other filesystem activities that might
      * affect the directory. The {@link #createDirectories createDirectories}
@@ -636,8 +637,9 @@ public static Path createFile(Path path, FileAttribute<?>... attrs)
      *          if the array contains an attribute that cannot be set atomically
      *          when creating the directory
      * @throws  FileAlreadyExistsException
-     *          if a directory could not otherwise be created because a file of
-     *          that name already exists <i>(optional specific exception)</i>
+     *          if a directory could not otherwise be created because a file or
+     *          directory of that name already exists
+     *          <i>(optional specific exception)</i>
      * @throws  IOException
      *          if an I/O error occurs or the parent directory does not exist
      */