Doc links for static variables

[Xanewok]

Originally created by @nrc (rust-dev-tools/rls-analysis#86):

cc rust-dev-tools/rls-analysis#85

Need to check if these exist, and that they work properly

[Xanewok]

Originally replied by @kzys (rust-dev-tools/rls-analysis#86 (comment)):

t, v, m and k are the suffixes Rustdoc has. I don't think we need a special handling for static variables.

https://github.com/rust-lang/rust/blob/aea1bd0/src/librustdoc/html/item_type.rs#L202

I double-checked with rust-lang/rust's docs.

% find build/x86_64-apple-darwin/doc/ -name '*.html' | perl -ne 'print "$1\n" if m/\.([a-z])\.html/' | sort | uniq -c
   3 k
  73 m
1294 t
4691 v
% find build/x86_64-apple-darwin/doc -name '*.html' | grep '\.k\.html'
build/x86_64-apple-darwin/doc/std/let.k.html
build/x86_64-apple-darwin/doc/std/struct.k.html
build/x86_64-apple-darwin/doc/std/fn.k.html
%