minor #9301 No more ambiguity on prod mode (juferchaud, javiereguiluz)

This PR was submitted for the 4.0 branch but it was merged into the 3.4 branch instead (closes #9301).

Discussion
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No more ambiguity on prod mode

Because APP_ENV/SYMFONY_ENV=prod is mandatory when you want to enable the prod environment, it should be explained somewhere, explicitly.

Commits
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97b296d Minor reword
3d7c3b3 No more ambiguity on prod mode

Author: javiereguiluz

deployment.rst

@@ -138,6 +138,11 @@ If your application uses environment variables instead of these parameters, you
 must define those env vars in your production server using the tools provided by
 your hosting service.
 
+At the very least you need to define the ``SYMFONY_ENV=prod`` (or
+``APP_ENV=prod`` if you're using :doc:`Symfony Flex </setup/flex>`) to run the
+application in ``prod`` mode, but depending on your application you may need to
+define other env vars too.
+
 C) Install/Update your Vendors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~