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).
Ahhh I see what you're saying. Also, the phrasing is awkward, should probably be "can be used to discard the result of a statement at".

What I was thinking of was that ; discards the results of the 1, which is an expression, and gives () instead.