minor #8122 Improved the explanation about deployment + parameters.yml (javiereguiluz)

This PR was squashed before being merged into the 2.7 branch (closes #8122).

Discussion
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Improved the explanation about deployment + parameters.yml

This fixes #5158.

As usual, the explanations in this article are a bit vague, because this is a generic deployment article and it contains no details (we have dedicated articles to explain those details).

Commits
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186458c Improved the explanation about deployment + parameters.yml

Author: xabbuh

deployment.rst

@@ -120,11 +120,20 @@ Check if your server meets the requirements by running:
 
     $ php app/check.php
 
-B) Configure your ``app/config/parameters.yml`` File
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+.. _b-configure-your-app-config-parameters-yml-file:
 
-This file should *not* be deployed, but managed through the automatic utilities
-provided by Symfony.
+B) Configure your Parameters File
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Most Symfony applications define configuration parameters in a file called
+``app/config/parameters.yml``. This file should *not* be deployed, because
+Symfony generates it automatically using the ``app/config/parameters.yml.dist``
+file as a template (that's why ``parameters.yml.dist`` must be committed and
+deployed).
+
+If your application uses environment variables instead of these parameters, you
+must define those env vars in your production server using the tools provided by
+your hosting service.
 
 C) Install/Update your Vendors
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~