Added clarification on runtime complexity for ListUtils.retainAll.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/collections/trunk@1377523 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/collections/ListUtils.java

@@ -276,6 +276,12 @@ public class ListUtils {
      * in <code>collection</code> unless <code>retain</code> does not contain <code>e</code>, in which
      * case the cardinality is zero. This method is useful if you do not wish to modify
      * the collection <code>c</code> and thus cannot call <code>collection.retainAll(retain);</code>.
+     * <p> 
+     * This implementation iterates over <code>collection</code>, checking each element in
+     * turn to see if it's contained in <code>retain</code>. If it's contained, it's added
+     * to the returned list. As a consequence, it is advised to use a collection type for
+     * <code>retain</code> that provides a fast (e.g. O(1)) implementation of
+     * {@link Collection#contains(Object)}.
      * 
      * @param <E>  the element type
      * @param collection  the collection whose contents are the target of the #retailAll operation