The current ebuild for cweb does not install the cwebmac.tex file. This file is required by TeX to create documentation from the files created by cweave. It should be installed into the teTeX tree.
Created attachment 3926 [details] ebuild to fix bug 7931 This ebuild installs all the files which would be installed if "make install" was run. It also runs "texconfig rehash" after installing the cweb macros in the texmf tree. I'm not sure if that step is strictly necessary, but if it is then this package should depend on app-text/tetex. Sorry for not adding that to the ebuild, but I only just thought of that now, and I think it may require some changes anyway. I've never submitted one of these before, but "it worked for me" :)
Ok, sorry for the delay. Included your ebuild (if crediting is wrong, let me know and I'll fix) and closing the bug. Thanks for your work!