Add details for pluralization on extra fields message

reference/forms/types/options/extra_fields_message.rst.inc

@@ -1,9 +1,15 @@
 ``extra_fields_message``
 ~~~~~~~~~~~~~~~~~~~~~~~~
 
+.. versionadded:: 5.1
+
+The support for pluralization was introduced in Symfony 5.1.
+
 **type**: ``string`` **default**: ``This form should not contain extra fields.``
 
 This is the validation error message that's used if the submitted form data
 contains one or more fields that are not part of the form definition. The
 placeholder ``{{ extra_fields }}`` can be used to display a comma separated
 list of the submitted extra field names.
+
+This message can be pluralized, see :ref:`formatting pluralized messages <component-translation-pluralization>` for details.

translation/message_format.rst

@@ -168,6 +168,9 @@ you to use literal text in the select statements:
     readable for translators and, as you can see in the ``other`` case, other
     parts of the sentence might be influenced by the variables.
 
+
+.. _component-translation-pluralization:
+
 Pluralization
 -------------