`missing_docs` lint should enforce documentation on associated types

Since they are part of the public API.

This currently compiles:

``` rust
#![crate_type = "lib"]
#![deny(missing_docs)]
#![feature(associated_types)]

//! Linear algebra

/// 2D array of elements
pub trait Matrix {
    // missing docs?
    type Elem;
}
```

`rustc 0.13.0-nightly (c7dd3c4d6 2015-01-05 23:51:00 +0000)`
