Add information about retrying on 429 error code

docs/src/main/sphinx/develop/client-protocol.md

@@ -25,8 +25,13 @@ links.
 If the client request returns an HTTP 502, 503, or 504, that means there was
 an intermittent problem processing request and the client should try again in
 50-100 ms. Trino does not generate those codes by itself, but those can be
-generated by gateways/load balancers in front of Trino. Any HTTP status other
-than 502, 503, 504 or 200 means that query processing has failed.
+generated by load balancers in front of Trino. 
+
+Additionally, if the request returns a 429 status code, the client should 
+retry the request using the `Retry-After` header value provided. 
+
+Any HTTP status other than 502, 503, 504 or 200 means that query processing 
+has failed.
 
 The `/v1/statement` `POST` request returns a JSON document of type
 `QueryResults`, as well as a collection of response headers. The `QueryResults`