feature #4269 [Cookbook][External Parameters] Enhance content (bicpi)

This PR was merged into the 2.3 branch.

Discussion
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[Cookbook][External Parameters] Enhance content

Enhance description of environment variables used as container parameters

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | -

Commits
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93684e2 [Cookbook][External Parameters] Enhance description of environment variables

cookbook/configuration/external_parameters.rst

@@ -14,9 +14,13 @@ Environment Variables
 ---------------------
 
 Symfony will grab any environment variable prefixed with ``SYMFONY__`` and
-set it as a parameter in the service container. Double underscores are replaced
-with a period, as a period is not a valid character in an environment variable
-name.
+set it as a parameter in the service container. Some transformations are 
+applied to the resulting parameter name:
+
+* ``SYMFONY__`` prefix is removed;
+* Parameter name is lowercased;
+* Double underscores are replaced with a period, as a period is not 
+  a valid character in an environment variable name.
 
 For example, if you're using Apache, environment variables can be set using
 the following ``VirtualHost`` configuration:
@@ -94,6 +98,14 @@ You can now reference these parameters wherever you need them.
             )
         ));
 
+.. note::
+
+    Even in debug mode, setting or changing an environment variable
+    requires your cache to be cleared to make the parameter available.
+    In debug mode, this is required since only a change to a configuration
+    file that is loaded by Symfony triggers your configuration to be
+    re-evaluated.
+
 Constants
 ---------