clarify what max_network_retries does

README.md

@@ -102,12 +102,15 @@ stripe.proxy = "https://user:pass@example.com:1234"
 ### Configuring Automatic Retries
 
 Number of automatic retries on requests that fail due to an intermittent
-network problem can be configured:
+problem can be configured:
 
 ```python
 stripe.max_network_retries = 2
 ```
 
+A request is retried if it times out or fails due to a network error or if the
+API responds with a `409` HTTP status code.
+
 [Idempotency keys][idempotency-keys] are automatically generated and added to
 requests, when not given, to guarantee that retries are safe.