minor #10396 Update search location of serializer configuration (robob4him)

This PR was squashed before being merged into the 4.1 branch (closes #10396).

Discussion
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Update search location of serializer configuration

(Docs change) Symfony 4.1 uses the 'serializer' directory, not 'serialization'.

Commits
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3988e2d Update search location of serializer configuration

Author: javiereguiluz

serializer.rst

@@ -137,6 +137,8 @@ In addition to the ``@Groups`` annotation, the Serializer component also
 supports YAML or XML files. These files are automatically loaded when being
 stored in one of the following locations:
 
+* All ``*.yaml`` and ``*.xml`` files in the ``config/serializer/``
+  directory.
 * The ``serialization.yaml`` or ``serialization.xml`` file in
   the ``Resources/config/`` directory of a bundle;
 * All ``*.yaml`` and ``*.xml`` files in the ``Resources/config/serialization/``