Document Kubernetes Kit backend session expiration policy

articles/tools/kubernetes/configuration.adoc

@@ -84,3 +84,24 @@ spring:
     host: redis-service
 ----
 --
+
+== Backend Session Expiration Policy
+
+Backend session expiration policy allows to define an expiration timeout for the backend stored session. By default no expiration is set, but can be expiration can be activated setting the property or by providing a custom SessionExpirationPolicy bean. If no unit is provided in the property, seconds are assumed.
+
+[.example]
+--
+.application.properties
+[source,properties]
+----
+vaadin.kubernetes.backend-session-expiration-tolerance: duration
+----
+
+.application.yaml
+[source,yaml]
+----
+vaadin:
+  kubernetes:
+    backend-session-expiration-tolerance: duration
+----
+--