rustdoc: s/nil/unit/ internally.

Quick poll on IRC suggested that unit was the preferred name for ().
core: add a primitive page for `()`.
auto merge of #15321 : huonw/rust/nil-prim, r=alexcrichton

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).