Tuple struct fields missing in documented signatures

E.g. a crate like this:

``` rust
#![crate_type="lib"]
#![crate_id="test"]

pub struct A(int);
pub struct B(pub int);
```

Produces the following signatures:

``` rust
pub struct A();
pub struct B();
```

I'd expect them to be something like:

``` rust
pub struct A (
    // some fields omitted
);
pub struct B (int);
```
