[doc] Document which opam package provides which modules

Now that js_of_ocaml's opam package has been split into several ones, it would be nice to document on the respective modules pages from which opam package they come from (at least for the optional modules).

Currently the only way I know is to run `opam search js_of_ocaml` and try to guess the mapping between `js_of_ocaml-*` opam packages and ocaml modules.