minor #10553 Mention the service tag of env processors (javiereguiluz)

This PR was merged into the 4.1 branch.

Discussion
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Mention the service tag of env processors

This continues the improvements of #10549.

Commits
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69f84e2 Mention the service tag of env processors

Author: javiereguiluz

configuration/external_parameters.rst

@@ -471,8 +471,7 @@ Custom Environment Variable Processors
 
 It's also possible to add your own processors for environment variables. First,
 create a class that implements
-:class:`Symfony\\Component\\DependencyInjection\\EnvVarProcessorInterface` and
-then, define a service for that class::
+:class:`Symfony\\Component\\DependencyInjection\\EnvVarProcessorInterface`::
 
     use Symfony\Component\DependencyInjection\EnvVarProcessorInterface;
 
@@ -493,6 +492,12 @@ then, define a service for that class::
         }
     }
 
+To enable the new processor in the app, register it as a service and
+:doc:`tag it </service_container/tags>` with the ``container.env_var_processor``
+tag. If you're using the
+:ref:`default services.yaml configuration <service-container-services-load-example>`,
+this is already done for you, thanks to :ref:`autoconfiguration <services-autoconfigure>`.
+
 Constants
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