Fix Javadoc warning

[WARNING]
...\commons-collections\src\main\java\org\apache\commons\collections4\list\TreeList.java:955:
warning: overridden methods do not document exception type
java.lang.NullPointerException (package java.lang class
NullPointerException)
[WARNING] * @throws NullPointerException {@inheritdoc}
[WARNING] ^

src/main/java/org/apache/commons/collections4/list/TreeList.java

@@ -952,7 +952,9 @@ public void add(final int index, final E obj) {
      *
      * @param c  the collection to be added to this list
      * @return {@code true} if this list changed as a result of the call
-     * @throws NullPointerException {@inheritDoc}
+     * @throws NullPointerException if the specified collection contains a
+     *         null element and this collection does not permit null elements,
+     *         or if the specified collection is null
      */
     @Override
     public boolean addAll(final Collection<? extends E> c) {