Where clause bounds in re-exported structs clauses aren't preserved by rustdoc

e.g. given:

``` rust
// foo.rs
pub struct Foo<A> where A: Copy;
pub struct Foo_<A: Copy>;
```

``` rust
// bar.rs
extern crate foo;

pub use foo::{Foo, Foo_};
```

rustdoc will show the bound on `bar::Foo_` but not on `bar::Foo`.
