better manpage info for the blackfilter

I have updated the manpage with more info about the blackfilter.
I don't know where to put the general description of the blackfilter. It currently is under the command --blackfilter-scan-direction (the first mention of the blackfilter in the manpage)
For the future I think it would be better if each component/filter of unpaper gets a dedicated description on how it works and the info for the commands becomes only one ore two lines.