core documentation is missing some primitive types found in std

[tomjakubowski]

Primitive types documented in std but not in core:

• bool http://doc.rust-lang.org/nightly/std/primitive.bool.html
• tuple http://doc.rust-lang.org/nightly/std/primitive.tuple.html

cc @steveklabnik

Related to #22084

[steveklabnik]

There's a few more of these as well.

[steveklabnik]

Oh, these aren't documented in core. core isn't going to be stable any time soon, so this isn't as high a priority as I thought.

[tomjakubowski]

It should be as straightforward as adding #![doc(primitive = "tuple")] to libcore/tuple.rs, and creating a new module core::bool that contains just the attribute #![doc(primitive = "bool")].

Core won't be stabilized anytime soon but I think it'll still get a lot of use from the heroic embedded and toy kernel developers still running on nightlies.