Document non-retryable 4xx polling retry behavior

Co-authored-by: Shri Sukhani <shrisukhani@users.noreply.github.com>

Author: cursoragent

README.md

@@ -141,6 +141,7 @@ Polling callback contracts are also validated:
 
 - Sync polling helpers require non-awaitable callback return values.
 - Async polling helpers require awaitable status/page/retry callbacks.
+- Polling retries skip non-retryable API client errors (HTTP `4xx`, except `429` rate-limit responses).
 
 Example: