feat(tvix/eval): Add docstrings as documentation for builtins

Add a new `documentation: Option<&'static str>` field to Builtin, and
populate it in the `#[builtins]` macro with the docstring of the builtin
function, if any.

Change-Id: Ic68fdf9b314d15a780731974234e2ae43f6a44b0
Reviewed-on: https://cl.tvl.fyi/c/depot/+/7205
Tested-by: BuildkiteCI
Reviewed-by: tazjin <tazjin@tvl.su>

tvix/eval/src/builtins/impure.rs

@@ -115,6 +115,7 @@ pub fn builtins_import(globals: &Weak<GlobalsMap>, source: SourceCode) -> Builti
             strict: true,
             name: "path",
         }],
+        None,
         move |mut args: Vec<Value>, vm: &mut VM| {
             let mut path = super::coerce_value_to_path(&args.pop().unwrap(), vm)?;
             if path.is_dir() {