minor #17101 [Serializer] Document support of PHP backed enumerations (94noni)

This PR was merged into the 5.4 branch.

Discussion
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[Serializer] Document support of PHP backed enumerations

Ref #15489
PS: I do not know if we should document that this requires `PHP v8.1+` and if yes, how to document it

Commits
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ffc4856 [Serializer] Document support of PHP backed enumerations

Author: javiereguiluz

components/serializer.rst

@@ -897,6 +897,14 @@ The Serializer component provides several built-in normalizers:
     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\\BackedEnumNormalizer`
+    This normalizer converts a \BackedEnum objects into strings or integers.
+
+.. versionadded:: 5.4
+
+    The ``BackedEnumNormalizer`` was introduced in Symfony 
+    5.4. PHP BackedEnum require at least PHP 8.1.
+
 :class:`Symfony\\Component\\Serializer\\Normalizer\\FormErrorNormalizer`
     This normalizer works with classes that implement
     :class:`Symfony\\Component\\Form\\FormInterface`.

serializer.rst

@@ -80,6 +80,12 @@ As well as the following normalizers:
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\ArrayDenormalizer`
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\ConstraintViolationListNormalizer`
 * :class:`Symfony\\Component\\Serializer\\Normalizer\\ProblemNormalizer`
+* :class:`Symfony\\Component\\Serializer\\Normalizer\\BackedEnumNormalizer`
+
+.. versionadded:: 5.4
+
+    :class:`Symfony\\Component\\Serializer\\Normalizer\\BackedEnumNormalizer`
+    was introduced in Symfony 5.4. PHP BackedEnum require at least PHP 8.1.
 
 Other :ref:`built-in normalizers <component-serializer-normalizers>` and
 custom normalizers and/or encoders can also be loaded by tagging them as