Document region changes

- Add new docs for regions changes related to StateMachineContext
  and StateConfigurer.
- Document new datajpamultipersist sample.
- Fixes #621

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

@@ -631,6 +631,38 @@ public class DocsConfigurationSampleTests extends AbstractStateMachineTests {
 	}
 // end::snippetP[]
 
+// tag::snippetPP[]
+	@Configuration
+	@EnableStateMachine
+	public class Config10RegionId
+			extends EnumStateMachineConfigurerAdapter<States2, Events> {
+
+		@Override
+		public void configure(StateMachineStateConfigurer<States2, Events> states)
+				throws Exception {
+			states
+				.withStates()
+					.initial(States2.S1)
+					.state(States2.S2)
+					.and()
+					.withStates()
+						.parent(States2.S2)
+						.region("R1")
+						.initial(States2.S2I)
+						.state(States2.S21)
+						.end(States2.S2F)
+						.and()
+					.withStates()
+						.parent(States2.S2)
+						.region("R2")
+						.initial(States2.S3I)
+						.state(States2.S31)
+						.end(States2.S3F);
+		}
+
+	}
+// end::snippetPP[]
+
 // tag::snippetQ[]
 	@Configuration
 	@EnableStateMachine