minor #7461 Update standards.rst (VolCh, javiereguiluz)

This PR was merged into the 3.2 branch.

Discussion
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Update standards.rst

According http://symfony.com/blog/new-in-symfony-3-2-runtime-environment-variables

Commits
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2867b34 Minor reword
68ae76f Update standards.rst

Author: xabbuh

contributing/code/standards.rst

@@ -218,7 +218,8 @@ Service Naming Conventions
 
 * The DI alias of the bundle is the first group (e.g. ``fos_user``);
 
-* Use lowercase letters for service and parameter names;
+* Use lowercase letters for service and parameter names (except when referring
+  to environment variables with the ``%env(VARIABLE_NAME)%`` syntax);
 
 * A group name uses the underscore notation.