Update condition

Author: liran2000

src/main/java/org/apache/commons/io/FileUtils.java

@@ -1180,6 +1180,7 @@ public static File delete(final File file) throws IOException {
      *
      * @param directory directory to delete
      * @throws IOException              in case deletion is unsuccessful
+     * @throws IllegalArgumentException if {@code directory} is not a directory
      */
     public static void deleteDirectory(final File directory) throws IOException {
         Objects.requireNonNull(directory, "directory");