One thing I wonder, actually - how feasible would it be to (in essence) do "symbolic execution" of #[cfg] directives? Specifically, have rustdoc "branch" on encountering a #[cfg] directive, generate documentation under both, and generate fragments for each such section? That could then allow selecting from a menu of #[cfg] options when reading the docs.
In terms of phrasing this as a vision statement, "Users of any platform should be able to easily read the documentation relevant to a crate as they would actually use it."