Update comment in to_string.

Author: Yarwin

godot-core/src/registry/callbacks.rs

@@ -192,14 +192,15 @@ pub unsafe extern "C" fn to_string<T: cap::GodotToString>(
     out_string: sys::GDExtensionStringPtr,
 ) {
     // Note: to_string currently always succeeds, as it is only provided for classes that have a working implementation.
-    // is_valid output parameter thus not needed.
 
     let storage = as_storage::<T>(instance);
     let instance = storage.get();
     let string = T::__godot_to_string(&*instance);
 
     // Transfer ownership to Godot
     string.move_into_string_ptr(out_string);
+
+    // Note: is_valid comes uninitialized and must be set.
     *is_valid = sys::conv::SYS_TRUE;
 }