[Internal] Documentation: Adds 429 mention on SDK design doc (#3476)

docs/SdkDesign.md

@@ -29,7 +29,8 @@ flowchart
  UserHandlers <--> DiagnosticHandler
  DiagnosticHandler <--> TelemetryHandler
  TelemetryHandler <--> RetryHandler[Cross region retries]
- RetryHandler <--> RouteHandler
+ RetryHandler <--> ThrottlingRetries[Throttling retries]
+ ThrottlingRetries <--> RouteHandler
  RouteHandler <--> IsPartitionedFeedOperation{{Partitioned Feed operation?}}
  IsPartitionedFeedOperation <-- No --> TransportHandler
  IsPartitionedFeedOperation <-- Yes --> InvalidPartitionExceptionRetryHandler
@@ -38,6 +39,12 @@ flowchart
  TransportHandler <--> TransportClient[[Selected Transport]]
 ```
 
+## Throttling retries
+
+Any HTTP response, with a status code `429` from the service means the current operation is being [rate limited](https://learn.microsoft.com/azure/cosmos-db/sql/troubleshoot-request-rate-too-large) and it's handled by the [RetryHandler](../Microsoft.Azure.Cosmos/src/Handler/RetryHandler.cs) through the [ResourceThrottleRetryPolicy](../Microsoft.Azure.Cosmos/src/ResourceThrottleRetryPolicy.cs).
+
+The policy will retry the operation using the delay indicated in the `x-ms-retryafter` response header up to the maximum configured in `CosmosClientOptions.MaxRetryAttemptsOnRateLimitedRequests` with a default value of 9.
+
 ## Cross region retries
 
 Any failure response from the Transport that matches the [conditions for cross-regional communication](https://docs.microsoft.com/azure/cosmos-db/troubleshoot-sdk-availability) are handled by the [RetryHandler](../Microsoft.Azure.Cosmos/src/Handler/RetryHandler.cs) through the [ClientRetryPolicy](../Microsoft.Azure.Cosmos/src/ClientRetryPolicy.cs).