docs(nodejs): clarify Table.add progress dispatch semantics

docs/src/js/interfaces/AddDataOptions.md

@@ -31,9 +31,9 @@ progress: (progress) => void;
 Optional callback invoked periodically with write progress.
 
 The callback is fired once per batch written and once more with
-`done: true` when the write completes. Calls are non-blocking — if the
-callback is slow, intermediate updates may be dropped to avoid stalling
-the write.
+`done: true` when the write completes. Calls are dispatched
+asynchronously to the JS event loop and never block the write — a slow
+callback will queue events rather than back-pressure the writer.
 
 Errors thrown from the callback are logged with `console.warn` and
 swallowed — they do not abort the write.