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).

diff --cc src/libcore/lib.rs
Simple merge

diff --cc src/librustdoc/clean/mod.rs
Simple merge

diff --cc src/libstd/lib.rs
Simple merge