[FrameworkBundle] Document assertResponseFormatSame()

Author: javiereguiluz

testing/functional_tests_assertions.rst

@@ -39,6 +39,12 @@ Response
 - ``assertResponseHasCookie()``
 - ``assertResponseNotHasCookie()``
 - ``assertResponseCookieValueSame()``
+- ``assertResponseFormatSame()`` (the response format is the value returned by
+  the :method:`Symfony\Component\HttpFoundation\Response::getFormat` method).
+
+.. versionadded:: 5.3
+
+    The ``assertResponseFormatSame()`` method was introduced in Symfony 5.3.
 
 Request
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