Rustdoc pretends that the struct is empty when it only has hidden public fields

``` rust
pub struct A { private: uint }
pub struct B { pub public: uint }
pub struct C { #[doc(hidden)] pub hidden: uint }
```

`A` correctly shows the "some fields omitted" comment, but `C` doesn't. (The recent example that exhibits this problem is `regex::Regex`). This is quite misleading.
