8024086: (fs) AtomicMoveNotSupportedException allows reason to be null

Modify javadoc specification of 'reason' parameter to allow null

Reviewed-by: alanb

jdk/src/java.base/share/classes/java/nio/file/AtomicMoveNotSupportedException.java

@@ -45,7 +45,7 @@ public class AtomicMoveNotSupportedException
      * @param   target
      *          a string identifying the target file or {@code null} if not known
      * @param   reason
-     *          a reason message with additional information
+     *          a reason message with additional information or {@code null}
      */
     public AtomicMoveNotSupportedException(String source,
                                            String target,