chore: document temporary workaround

src/Lean/Message.lean

@@ -305,6 +305,8 @@ macro_rules
   let r ← Lean.Syntax.expandInterpolatedStrChunks chunks (fun a b => `($a ++ $b)) (fun a => `(toMessageData $a))
   `(($r : MessageData))
 
+-- TODO: interpreter should not compiled code when building stdlib.
+-- The following instances cannot be defined before `syntax` because it would change the internal syntax node kinds and break the build.
 instance {α} [ToMessageData α] : ToMessageData (Option α) := ⟨fun | none => "none" | some e => "some ({toMessageData e})"⟩
 instance : ToMessageData (Option Expr) := ⟨fun | none => "<not-available>" | some e => toMessageData e⟩