Docs: Tooltips on hover for types from other crates

[casey]

I was looking at docs for tokio-proto, and noticed that there were helpful tooltips for types defined in the crate, but not for types like Handle and FramedIo, whice are defined in other crates.

It would be great if, even if they aren't clickable, types outside of the crate would get tooltips showing the full path of the type, to hint at where to look to learn more. So Handle would show a tooltip with tokio_core::reactor::Handle and FramedIo would show one with tokio_core::io::FramedIo.

[bluss]

FramedIo would be clickable if the crate that defines it sets the #![doc(html_root_url)] attribute. (Or if its docs are generated at the same time).

[casey]

Having them be clickable would be great, but even if not, could they still have tooltips?

[jyn514]

Triage: this still happens today.

[GuillaumeGomez]

It was fixed in #107242.