dev-ml/core_extended has a "doc" USE flag that builds and installs documentation. dev-ml/core doesn't. The fix is to add OASIS_BUILD_DOCS=1 before inherit oasis in the ebuild. This is fairly important since the "official" documentation is for an ancient version of core http://www.janestreet.com/ocaml/janestreet-ocamldocs/core/index.html
fixed, thanks