Add primitive doc for ()

This adds a primitive page for () like http://doc.rust-lang.org/master/std/uint/primitive.uint.html .

I would prefer the modules to be `std::tuple::unit`, but rustdoc only searches at the top level (filed as #15320).
Oh, don't know.

(It's mainly just a habit I have from mathematics, where even an "broken out" equation is normally written to be a natural part of the sentence it's in

> Hence, we've proven that
> 
>     a + 2 = 3
>
> where `a = 1`.

This may not be the style we want for Rust's docs.)