rustdoc: Add reexport info

[GuillaumeGomez]

It looks like this:

For this code:

#[doc(inline)]
pub use std::vec::Vec as integer_32;

So now come a few questions:

• Should we do it for local items too?
• What should we do for reexported doc(hidden) items?
• What should we go for reexported items within a doc(hidden) module as ancestor?
• Should we add an attribute to enable/disable this feature?

As this is likely going to need a lot of discussion, it's a draft for now. Nominating it for discussing it in next rustdoc meeting.

cc @rust-lang/rustdoc