rustdoc or rustc: duplicate impls for std::marker::Send

Hi everybody.

Look at https://doc.rust-lang.org/nightly/std/marker/trait.Send.html

There are
```rust
impl<T: ?Sized> !Send for Rc<T>
impl<T: ?Sized> !Send for Weak<T>
```
and
```rust
impl<T> !Send for Rc<T> where T: ?Sized
impl<T> !Send for Weak<T> where T: ?Sized
```

They are equal. Wow. There are same lines at the beginning.