Trait impls on type alias aren't documented on their underlying type

[aDotInTheVoid]

pub struct A;
pub type B = A;

pub trait T {}
impl T for B {}

This effects json and html

@rustbot modify labels: +C-Bug +T-rustdoc +A-rustdoc-json

[aDotInTheVoid]

pub struct A;
pub type B = A;

impl B {
    pub fn boo() {}
}

is handled correctly by html and json

[camelid]

I'm going to remove the A-rustdoc-json label since this is not a bug with JSON; it is likely a bug with clean or similar that then gets outputted as HTML or JSON.