rustdoc: Fn trait sugar output types are broken for inlined signatures

For example, see: http://doc.rust-lang.org/std/collections/struct.Bitv.html#method.from_fn

``` rust
fn from_fn<F>(len: usize, f: F) -> Bitv 
where F: FnMut(usize), <F as FnMut(usize)>::Output == bool
```

Should look like:

``` rust
fn from_fn<F>(len: usize, f: F) -> Bitv 
where F: FnMut(usize) -> bool
```

This is because of the change to use associated types for the output.
