Allow linking to module documentation

[Wilfred]

This is common in stdlib docs:

Ok is a enumvariant defined in libcore/result.rs.

    Ok(#[stable(feature = "rust1", since = "1.0.0")] T),

Result is a type that represents either success (Ok) or failure (Err).

See the std::result module documentation for details.

racer can access the std:result module documentaiton, so we should set up a link.