[FrameworkBundle] Document assertResponseFormatSame()

Author: javiereguiluz

testing/functional_tests_assertions.rst

@@ -39,6 +39,13 @@ Response
 - ``assertResponseHasCookie()``
 - ``assertResponseNotHasCookie()``
 - ``assertResponseCookieValueSame()``
+- ``assertResponseFormatSame()``, it returns the format associated to the MIME
+  type of the ``Content-Type`` of the response (e.g. it returns ``jsonld`` for
+  ``application/ld+json`` content type).
+
+.. versionadded:: 5.x
+
+    The ``assertResponseFormatSame()`` method was introduced in Symfony 5.3.
 
 Request
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