IO-724 deleteDirectory exception javadoc inaccurate update (#245)

* IO-724 deleteDirectory exception javadoc update

FileUtils.deleteDirectory javadoc is inaccurate for nonexistent directory

* Remove NullPointerException from javadoc

Remove NullPointerException from javadoc as future annotation may comes to place

* Update condition

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

@@ -1180,7 +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} does not exist or is not a directory
+     * @throws IllegalArgumentException if {@code directory} is not a directory
      */
     public static void deleteDirectory(final File directory) throws IOException {
         Objects.requireNonNull(directory, "directory");