Doc links for primitives

[Xanewok]

Originally created by @nrc (rust-dev-tools/rls-analysis#87):

Need to be primitive rather than type. cc rust-dev-tools/rls-analysis#85

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

[Xanewok]

Originally replied by @kzys (rust-dev-tools/rls-analysis#87 (comment)):

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.