Update docs

- Relates to #194

spring-statemachine-core/src/test/java/org/springframework/statemachine/docs/DocsConfigurationSampleTests.java

@@ -753,6 +753,54 @@ public class DocsConfigurationSampleTests extends AbstractStateMachineTests {
 	}
 // end::snippetU[]
 
+// tag::snippetUU[]
+	@Configuration
+	@EnableStateMachine
+	static class Config21 extends StateMachineConfigurerAdapter<String, String> {
+
+		@Override
+		public void configure(StateMachineStateConfigurer<String, String> states)
+				throws Exception {
+			states
+			.withStates()
+				.initial("S1")
+				.state("S2")
+				.state("S3")
+				.and()
+				.withStates()
+					.parent("S2")
+					.initial("S21")
+					.entry("S2ENTRY")
+					.exit("S2EXIT")
+					.state("S22");
+		}
+
+		@Override
+		public void configure(StateMachineTransitionConfigurer<String, String> transitions)
+				throws Exception {
+			transitions
+			.withExternal()
+				.source("S1").target("S2")
+				.event("E1")
+				.and()
+			.withExternal()
+				.source("S1").target("S2ENTRY")
+				.event("ENTRY")
+				.and()
+			.withExternal()
+				.source("S22").target("S2EXIT")
+				.event("EXIT")
+				.and()
+			.withEntry()
+				.source("S2ENTRY").target("S22")
+				.and()
+			.withExit()
+				.source("S2EXIT").target("S3");
+		}
+	}
+
+// end::snippetUU[]
+
 	@Configuration
 	@EnableStateMachine
 	public class Config16