Created attachment 303779 [details, diff] patch Making html documentation of nanoc requires dev-ruby/yard and dev-ruby/rdiscount. Here is patch.
It looks like dev-ruby/kramdown is also mandatory to build the docs. Did you miss that in your patch? Unfortunately kramdown is not keyworded for x86 so I can't fix this right away.
Fixed in CVS, thanks. I cannot confirm rdiscount is needed for docs generation, worked just fine without it. yard and kramdown are added.