support for function (or variable) documentation

I'm using this lsp through https://github.com/XadillaX/vscode-language-viml and from my testing there and the screenshots here it seems that there is documentation information shown for in-built functions like `expand()`, but not for user-defined functions (from the README example: the ´echo_message()´ function.

Could this be added, possibly also for variables (much less needed than for user defined functions, but still useful)?