Note that env vars are not always compatible with options

There are cases where environment variables cannot be used in
place of regular configuration options. This commit makes the limitation
explicit.

From symfony/symfony#39902

Author: melkamar

configuration.rst

@@ -602,6 +602,10 @@ You can reference environment variables using the special syntax
 ``%env(ENV_VAR_NAME)%``. The values of these options are resolved at runtime
 (only once per request, to not impact performance).
 
+Note that not all config options are compatible with environment variables. There are
+`cases <https://github.com/symfony/symfony/issues/39902>`_ which may require refactoring
+the config definition in order to work with environment variables.
+
 This example shows how you could configure the database connection using an env var:
 
 .. configuration-block::