document generic synthetic bean creation

docs/src/main/asciidoc/writing-extensions.adoc

@@ -310,6 +310,25 @@ SyntheticBeanBuildItem syntheticBean(TestRecorder recorder) {
 ----
 <1> The "real" components can inject the `TestContext` directly.
 
+
+It is also possible to create a generic synthetic bean `Foo<Bar>`.
+
+[source%nowrap,java]
+----
+@BuildStep
+@Record(STATIC_INIT)
+SyntheticBeanBuildItem syntheticBean(TestRecorder recorder) {
+   return SyntheticBeanBuildItem.configure(Foo.class)
+                .types(ParameterizedType.create(Foo.class, ClassType.create(Bar.class)))) <1>
+                .scope(Singleton.class)
+                .runtimeValue(recorder.createFooBar())
+                .done();
+}
+----
+
+<1> types must be used to specify the generic type.
+
+
 === Some types of extensions
 
 There exist multiple stereotypes of extension, let's list a few.