save-analysis data from rustup does not contain doc information for type aliases

[Xanewok]

Originally created by @QuietMisdreavus (rust-dev-tools/rls-analysis#65):

If I look for io::Result I can properly find std::io::error::Result to match it, but it doesn't seem to have the documentation text for it. I would expect it to have the documentation that can be seen here, but the string def.docs is empty.