[Validator] Add the fields option to the Unique constraint

Author: wkania

reference/constraints/Unique.rst

@@ -103,6 +103,20 @@ strings:
 Options
 -------
 
+``fields``
+~~~~~~~~~~
+
+**type**: ``array`` | ``string``
+
+
+.. versionadded:: 6.1
+
+    The ``fields`` option was introduced in Symfony 6.1.
+
+This is optional and defines key or keys
+in a collection that should be checked for uniqueness.
+By default, all collection keys are checked for uniqueness.
+
 .. include:: /reference/constraints/_groups-option.rst.inc
 
 ``message``