[LANG-653] Updated user guide

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1026486 13f79535-47bb-0310-9956-ffa450edef68

src/site/xdoc/userguide.xml

@@ -255,6 +255,14 @@ public interface ConcurrentInitializer<T> {
      obtained. There are different implementations of the interface available
      addressing various use cases:
    </p>
+   <p>
+     <code>ConstantInitializer</code> is a very straightforward implementation of
+     the <code>ConcurrentInitializer</code> interface: An instance is passed an
+     object when it is constructed. In its <code>get()</code> method it simply
+     returns this object. This is useful, for instance in unit tests or in cases
+     when you want to pass a specific object to a component which expects a
+     <code>ConcurrentInitializer</code>.
+   </p>
    <p>
      The <code>LazyInitializer</code> class can be used to defer the creation of
      an object until it is actually used. This makes sense, for instance, if the