Doc system not working for signature-dependent docstrings

quoting @MichaelHatherly from https://github.com/JuliaLang/julia/pull/20264#issuecomment-275644314

> Passing `sig` should allow for returning different docs for `?x(1)` and `?x(1.0)`, currently it seems there's a bug in `?` mode preventing that from working though.