[Minor] add documentation about custom search expression on website

li.strolch.agent/src/main/java/li/strolch/search/SearchExpression.java

@@ -10,6 +10,7 @@ import li.strolch.model.activity.Activity;
  *
  * @param <T>
  */
+@FunctionalInterface
 public interface SearchExpression<T extends StrolchRootElement> {
 
 	/**

li.strolch.website/www.strolch.li/documentation-searches.html

@@ -120,6 +120,11 @@ search.where(id(isEqualTo("myId")) //
         <p>Note how the predicates can be chained to the search expression, or passed as a parameter to the
            expression.</p>
 
+        <p>In addition to using predefined search search expressions, one can also just pass a lambda expression which
+           performs a custom filter:</p>
+        <pre>
+personSearch.where(person -> person.getName().length() == 3);</pre>
+
         <p>See the
             <a href="https://github.com/4treesCH/strolch/blob/develop/li.strolch.agent/src/test/java/li/strolch/search/StrolchSearchTest.java"
                target="_blank">StrolchSearchTest</a> for many ways in which you can implement tests.</p>