Some dataflows' options are not covered in the docs

[roll]

Overview

If I got it right the idea behind extra options for some processors is to provide access for lower level libs like tabulator (load options etc). Having a link to external docs for these cases seems very reasonable.

On the other hand, dataflows options I think need to be covered here too or maybe some unified processors documentation could be created. Otherwise it's getting too much layers for finding documentation.

My problem was with cast_strategy (also I'm not sure how to use it properly in dpp because it uses constants in df)