fix(tvix/eval): fix doc comment syntax where applicable

As pointed out by grfn on cl/6091

Change-Id: I28308577b7cf99dffb4a4fd3cc8783eb9ab4d0d6
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6460
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -36,6 +36,9 @@ macro_rules! force {
     };
 }
 
+/// Return all pure builtins, that is all builtins that do not rely on
+/// I/O outside of the VM and which can be used in any contexts (e.g.
+/// WASM).
 fn pure_builtins() -> Vec<Builtin> {
     vec![
         Builtin::new("add", 2, |mut args, _| {