Rustdoc for `impl Trait` in argument position is broken

```rust
pub fn foo(_x: impl Clone, _y: i32, _z: impl Copy) {
}
```

Running rustdoc on this code generates the following documentation:

![](https://user-images.githubusercontent.com/7091080/37831646-a61413c6-2ee9-11e8-8ec2-a6137956d922.png)