documented the use of argThat. fixes #240

guide/src/test/scala/org/specs2/guide/Matchers.scala

@@ -500,6 +500,10 @@ m.get(999) must_== "element"
 m.get(===(123)) returns "one"
 }}
 
+**Note** the call above works because there is an implicit method `argThat` which transforms a specs2 `Matcher[T]` into a Hamcrest one and in turn call Mockito's `org.mockito.Matchers.argThat` method to register the Hamcrest matcher. However [sometimes](https://groups.google.com/forum/#!msg/specs2-users/_slOZQoICzU/DF-ZQCq_GmkJ) the implicit conversion is not called and you have to explicitly call the `argThat` method like so: ${snippet{
+  m.get(argThat(===(123))) returns "one"
+}}
+
 ##### Callbacks
 
 In some rare cases, it is necessary to have the return value depend on the parameters passed to the mocked method: ${snippet{