feature #4645 Remove note that's no longer the case (thewilkybarkid)

This PR was merged into the 2.3 branch.

Discussion
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Remove note that's no longer the case

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3
| Fixed tickets |

symfony/symfony#12972 changes the behaviour so that this note is no longer required.

Commits
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de8b547 Remove note that's no longer the case

cookbook/configuration/external_parameters.rst

@@ -98,14 +98,6 @@ 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
 ---------