Add docs for state actions

- Relates to #238

docs/src/reference/asciidoc/sm.adoc

@@ -190,6 +190,7 @@ needs more clarification:
 * We defined action for initial state `S1`.
 * We defined entry action for state `S1` and left exit action empty.
 * We defined exit action for state `S2` and left entry action empty.
+* We defined a single state action for state `S2`.
 * We defined entry action as well as exit action for state `S3`.
 * Notice how state `S1` is used twice with `initial()` and `state()`
   functions. This is only needed if you want to define entry or exit
@@ -204,6 +205,19 @@ with `state()` is then executed if state machine is transitioning back
 and forward between initial and non-inital states.
 ====
 
+==== State Actions
+
+State actions are executed differently compared to entry and exit
+actions simply because execution happens after state has been entered
+and can be cancelled if state exit happens before particular action
+has been completed.
+
+State Actions are executed using a normal Spring `TaskScheduler`
+wrapped within a `Runnable` which may get cancelled via
+`ScheduledFuture`. What this means is that whatever your doing in your
+action, you need to be able to catch `InterruptedException` which is
+raised if task is cancelled.
+
 === Configuring Pseudo States
 
 _Pseudo state_ configuration is usually done by configuring states and

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

@@ -247,6 +247,7 @@ public class DocsConfigurationSampleTests extends AbstractStateMachineTests {
 						.initial(States.S1, action())
 						.state(States.S1, action(), null)
 						.state(States.S2, null, action())
+						.state(States.S2, action())
 						.state(States.S3, action(), action());
 			}