Document native/bytecode difference

toplevel/native/topeval.ml

@@ -169,7 +169,15 @@ let execute_phrase print_outcome ppf phr =
       let sg' = Typemod.Signature_names.simplify newenv names sg in
       ignore (Includemod.signatures oldenv ~mark:Mark_positive sg sg');
       Typecore.force_delayed_checks ();
-      (* The expression is "named" after typing in order to ensure that both
+      (* `let _ = <expression>` or even just `<expression>` require special
+         handling in toplevels, or nothing is displayed. In bytecode, the
+         lambda for <expression> is directly executed and the result _is_ the
+         value. In native, the lambda for <expression> is compiled and loaded
+         from a DLL, and the result of loading that DLL is _not_ the value
+         itself. In native, <expression> must therefore be named so that it can
+         be looked up after the DLL has been dlopen'd.
+
+         The expression is "named" after typing in order to ensure that both
          bytecode and native toplevels always type-check _exactly_ the same
          expression. Adding the binding at the parsetree level (before typing)
          can create observable differences (e.g. in type variable names, see