Merge branch '6.4' into 7.3

* 6.4:
  Minor reword
  Add a note that checking for flash messages defeats HTTP cacheability

Author: javiereguiluz

session.rst

@@ -265,6 +265,17 @@ It's common to use ``notice``, ``warning`` and ``error`` as the keys of the
 different types of flash messages, but you can use any key that fits your
 needs.
 
+.. tip::
+
+    Accessing flash messages requires starting the session, which in turn
+    causes Symfony to mark the response as ``private``. In general, because
+    flash messages are meant to be displayed only once, pages that might show
+    them cannot reasonably be cached by HTTP caches.
+
+    As an alternative, you can load flash messages asynchronously through
+    another HTTP request (for example, using a `Twig Live Component`_), making
+    the original page fully cacheable.
+
 Configuration
 -------------
 
@@ -1881,6 +1892,7 @@ the example below:
     can use the Symfony save handler without side effects and that the session
     has not been started before Symfony is initialized.
 
+.. _`Twig Live Component`: https://symfony.com/bundles/ux-live-component/current/index.html
 .. _`phpredis extension`: https://github.com/phpredis/phpredis
 .. _`DoctrineMongoDBBundle configuration`: https://symfony.com/doc/master/bundles/DoctrineMongoDBBundle/config.html
 .. _`MongoDB shell`: https://docs.mongodb.com/manual/mongo/