Harmonize Spring Session flush mode properties javadoc

See gh-17797

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/HazelcastSessionProperties.java

@@ -35,7 +35,8 @@ public class HazelcastSessionProperties {
 	private String mapName = "spring:session:sessions";
 
 	/**
-	 * Sessions flush mode.
+	 * Sessions flush mode. Determines when session changes are written to the session
+	 * store.
 	 */
 	private FlushMode flushMode = FlushMode.ON_SAVE;
 

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/session/RedisSessionProperties.java

@@ -37,7 +37,8 @@ public class RedisSessionProperties {
 	private String namespace = "spring:session";
 
 	/**
-	 * Sessions flush mode.
+	 * Sessions flush mode. Determines when session changes are written to the session
+	 * store.
 	 */
 	private FlushMode flushMode = FlushMode.ON_SAVE;