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, yes, the phrasing is horrible. I edited two sentences into one without actually making them connect properly...

Yeah, that's a good point, I think saying "end of some block" specifically is enough to change statement to expression, so I'll do that.