minor #8470 Mention that public services must add an alias to their classes (javiereguiluz)

This PR was squashed before being merged into the 3.3 branch (closes #8470).

Discussion
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Mention that public services must add an alias to their classes

This fixes #7968.

Commits
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caf045b Fixed syntax
5578ab0 Mention that public services must add an alias to their classes

Author: weaverryan

contributing/code/standards.rst

@@ -222,7 +222,10 @@ Service Naming Conventions
 * 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.
+* A group name uses the underscore notation;
+
+* Add class aliases for public services (e.g. alias ``Symfony\Component\Something\ClassName``
+  to ``something.service_name``).
 
 Documentation
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