minor #9855 Mention the new ConstraintViolationListNormalizer (javiereguiluz)

This PR was squashed before being merged into the 4.1 branch (closes #9855).

Discussion
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Mention the new ConstraintViolationListNormalizer

This fixes #9480.

Commits
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0d00129 Mention the new ConstraintViolationListNormalizer

Author: javiereguiluz

components/serializer.rst

@@ -612,6 +612,14 @@ There are several types of normalizers available:
     This normalizer converts :phpclass:`DateInterval` objects into strings.
     By default it uses the ``P%yY%mM%dDT%hH%iM%sS`` format.
 
+:class:`Symfony\\Component\\Serializer\\Normalizer\\ConstraintViolationListNormalizer`
+    This normalizer converts objects that implement
+    :class:`Symfony\\Component\\Validator\\ConstraintViolationListInterface`
+    into a list of errors according to the `RFC 7807`_ standard.
+
+    .. versionadded:: 4.1
+        The ``ConstraintViolationListNormalizer`` was introduced in Symfony 4.1.
+
 .. _component-serializer-encoders:
 
 Encoders
@@ -1255,3 +1263,4 @@ Learn more
 .. _XML: https://www.w3.org/XML/
 .. _YAML: http://yaml.org/
 .. _CSV: https://tools.ietf.org/html/rfc4180
+.. _`RFC 7807`: https://tools.ietf.org/html/rfc7807