Polish SessionRepository javadoc

Clarifies that getSession updates expiration, but it must be saved for
the expiration to be persisted.

spring-session/src/main/java/org/springframework/session/SessionRepository.java

@@ -48,6 +48,13 @@ public interface SessionRepository<S extends Session> {
 
     /**
      * Gets the {@link Session} by the {@link Session#getId()} or null if no {@link Session} is found.
+     *
+     * <p>
+     * If the {@link Session} extends {@link ExpiringSession}, then {@link ExpiringSession#getLastAccessedTime()} will be
+     * updated on the returned object. In order to persist this change, {@link #save(Session)} must be invoked on the returned
+     * instance.
+     * </p>
+     *
      * @param id the {@link org.springframework.session.Session#getId()} to lookup
      * @return the {@link Session} by the {@link Session#getId()} or null if no {@link Session} is found.
      */