minor #18850 [Config] [Configuration] Emphasize env vars purpose (alexandre-daubois)

This PR was merged into the 5.4 branch.

Discussion
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[Config] [Configuration] Emphasize env vars purpose

Fix #14745

Commits
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eab31f0 [Configuration] Emphasize env vars purpose

Author: javiereguiluz

configuration.rst

@@ -599,6 +599,15 @@ The values of these options are resolved at runtime (only once per request, to
 not impact performance) so you can change the application behavior without having
 to clear the cache.
 
+.. note::
+
+    Environment variables are designed to store information that can
+    dynamically change in a production environment, such as an API
+    key that may have an expiration date. This information could be updated
+    without having to redeploy the application. Thus, not everything is a good
+    candidate for env vars, and
+    :ref:`parameters <configuration-parameters>` should be used in other cases.
+
 This example shows how you could configure the application secret using an env var:
 
 .. configuration-block::