minor symfony#12066 [Configuration] Simplified how default env vars are defined (javiereguiluz)

This PR was squashed before being merged into the 4.3 branch (closes symfony#12066).

Discussion
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[Configuration] Simplified how default env vars are defined

Changes according to @nicolas-grekas' comments in https://twitter.com/arnolanglade/status/1156107440775192576

Commits
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9581c06 [Configuration] Simplified how default env vars are defined

Author: javiereguiluz

configuration.rst

@@ -488,33 +488,12 @@ This example shows how to configure the database connection using an env var:
 The next step is to define the value of those env vars in your shell, your web
 server, etc. This is explained in the following sections, but to protect your
 application from undefined env vars, you can give them a default value using the
-``parameters`` key:
+``.env`` file:
 
-.. configuration-block::
-
-    .. code-block:: yaml
-
-        # config/services.yaml
-        parameters:
-            env(DATABASE_URL): 'sqlite:///%kernel.project_dir%/var/data.db'
-
-    .. code-block:: xml
-
-        <!-- config/services.xml -->
-        <?xml version="1.0" encoding="UTF-8" ?>
-        <container xmlns="http://symfony.com/schema/dic/services"
-            xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-            xsi:schemaLocation="http://symfony.com/schema/dic/services https://symfony.com/schema/dic/services/services-1.0.xsd">
-
-            <parameters>
-                <parameter key="env(DATABASE_URL)">sqlite:///%kernel.project_dir%/var/data.db</parameter>
-            </parameters>
-        </container>
-
-    .. code-block:: php
+.. code-block:: bash
 
-        // config/services.php
-        $container->setParameter('env(DATABASE_URL)', 'sqlite:///%kernel.project_dir%/var/data.db');
+    # .env
+    DATABASE_URL=sqlite:///%kernel.project_dir%/var/data.db
 
 .. seealso::