minor #21381 Add a note that checking for flash messages defeats HTTP cacheability (mpdude)

This PR was squashed before being merged into the 6.4 branch.

Discussion
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 Add a note that checking for flash messages defeats HTTP cacheability

This may be surprising when HTTP cache performance is a concern, so let's give this early heads-up notice.

Commits
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99e0181  Add a note that checking for flash messages defeats HTTP cacheability

Author: javiereguiluz

session.rst

@@ -265,6 +265,13 @@ 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 means that the session must be started, which
+    in turn will cause Symfony to mark the responses as ``private``. In general,
+    since flashes shall displayed only once, pages that might display flashes
+    cannot reasonably be cached in HTTP caches.
+
 Configuration
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