Note about keyword-only fail parameter in StopDownload docs

docs/topics/exceptions.rst

@@ -97,13 +97,15 @@ response:
 
 * If ``fail=False``, the request callback is called instead.
 
-Note that ``fail`` is a keyword-only parameter.
-
 In both cases, the response could have its body truncated: the body contains
 all bytes received up until the exception is raised, including the bytes
-received in the signal handler that raises the exception. Also, the response 
-object is marked with ``"download_stopped"`` in its :attr:`Response.flags` 
+received in the signal handler that raises the exception. Also, the response
+object is marked with ``"download_stopped"`` in its :attr:`Response.flags`
 attribute.
 
+.. note:: ``fail`` is a keyword-only parameter, i.e. raising
+    ``StopDownload(False)`` or ``StopDownload(True)`` will raise
+    a :class:`TypeError`.
+
 See the documentation for the :class:`~scrapy.signals.bytes_received` signal
 and the :ref:`topics-stop-response-download` topic for additional information and examples.