auto merge of #18822 : scialex/rust/better-rustdoc, r=alexcrichton

Changed `rustdoc` so that if we do not have the `strip-private` pass
enabled private modules will be included in the generated documentation

I added this because it is useful to be able to read the documentation in the very nice `rustdoc` web interface when doing internal work on a project. In this case we want the `rustdoc` to include modules that are hidden from consumers. Since this is not currently possible I added it here.