added short RFPDupeFilter.request_fingerprint interface description

docs/topics/settings.rst

@@ -410,7 +410,9 @@ The class used to detect and filter duplicate requests.
 The default (``RFPDupeFilter``) filters based on request fingerprint using
 the ``scrapy.utils.request.request_fingerprint`` function. In order to change
 the way duplicates are checked you could subclass ``RFPDupeFilter`` and
-override its ``request_fingerprint`` method.
+override its ``request_fingerprint`` method. This method should accept
+scrapy :class:`~scrapy.http.Request` object and return its fingerprint
+(a string).
 
 .. setting:: DUPEFILTER_DEBUG