Doc links for primitives

[nrc]

Need to be primitive rather than type. cc #85

Can probably be hacked fairly easily by hard-coding a list of primitives.

[kzys]

Don't we need primitives on save-analysis's JSON data?

RLS uses span-based methods such as https://github.com/rust-lang-nursery/rls/blob/377081d6cb14641affbc4dea6defa01984ae4a96/src/actions/hover.rs#L890. We may need to have a way to convert spans to primitives' defs, I think.