minor #11098 Some cache files might be written in runtime (cvdwel)

This PR was submitted for the master branch but it was merged into the 4.2 branch instead (closes #11098).

Discussion
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Some cache files might be written in runtime

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Commits
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3fcb22c Some cache files might be written in runtime

Author: javiereguiluz

setup/file_permissions.rst

@@ -9,7 +9,9 @@ was writable. But that is no longer true! In Symfony 4, everything works automat
 
 * In the ``prod`` environment (i.e. when ``APP_ENV`` is ``prod`` and ``APP_DEBUG``
   is ``0``), as long as you run ``php bin/console cache:warmup``, no cache files
-  will need to be written to disk at runtime.
+  will need to be written to disk at runtime. An exception to this is any type of
+  runtime cache, for example a query result cache or DoctrineCacheBundle with a
+  file_system cache provider configured.
 
 * In all environments, the log directory (``var/log/`` by default) must exist
   and be writable by your web server user and terminal user. One way this can