I'm not sure I can write a proper documentation (I think my language skill is not high enough). What I was thinking of is adding a sentense to http://doc.scrapy.org/en/latest/topics/settings.html#dupefilter-class:

> In order to change the way duplicates are checked you could subclass `RFPDupeFilter` and override it `request_fingerprint` method.

Please tell me if I should somehow commit this (or make another PR for documentation, or maybe? update??? current PR), or someone else will edit documentation more properly.