Document Kubernetes Kit backend session expiration policy

articles/tools/kubernetes/configuration.adoc

@@ -84,3 +84,34 @@ spring:
     host: redis-service
 ----
 --
+
+== Backend Session Expiration Policy
+
+The backend session expiration policy allows you to define an expiration timeout for the backend stored session. By default no expiration is set, but expiration can be activated by setting the `vaadin.kubernetes.backend-session-expiration-tolerance` property or by providing a custom `SessionExpirationPolicy` bean. The property defines the amount of time to be added to the HTTP session timeout to determine the expiration of the backend session. If time unit is not given, milliseconds is assumed.
+
+[.example]
+--
+.application.properties
+[source,properties]
+----
+# Given an HTTP session timeout of 30 minuts, the session stored in the backed will expire 35 minutes of inactivity
+vaadin.kubernetes.backend-session-expiration-tolerance: 5m
+----
+
+.application.yaml
+[source,yaml]
+----
+vaadin:
+  kubernetes:
+    backend-session-expiration-tolerance: duration
+----
+
+.Java
+[source,java]
+----
+@Bean
+SessionExpirationPolicy sessionExpirationPolicy() {
+    return sessionTimeout -> Duration.ofMinutes(60);
+}
+----
+--