Builtin types are not documented

[emberian]

Due to how the AST works, and the fact that they aren't defined in any crate, the builtin types do not show up as hyperlinkable in rustdoc output, and there is generally no way to discover what methods they implement.

They need to be special cased to make it easier to figure out what operations one can do on a &str or ~[T].

[emberian]

I think this is very important for 1.0, as without this the std docs are fairly crippled.

[brson]

Dupe of #14474