Update README.md (Azure#22076)

Author: gahl-levy

sdk/cosmos/azure-cosmos/README.md

@@ -433,7 +433,7 @@ print(json.dumps(container_props['defaultTtl']))
 
 For more information on TTL, see [Time to Live for Azure Cosmos DB data][cosmos_ttl].
 
-### Using the asynchronous client
+### Using the asynchronous client (Preview)
 
 The asynchronous cosmos client is a separate client that looks and works in a similar fashion to the existing synchronous client. However, the async client needs to be imported separately and its methods need to be used with the async/await keywords.
 
@@ -483,7 +483,7 @@ async with CosmosClient(URL, credential=KEY) as client: # the with statement wil
         )
 ```
 
-### Queries with the asynchronous client
+### Queries with the asynchronous client (Preview)
 
 Unlike the synchronous client, the async client does not have an `enable_cross_partition` flag in the request. Queries without a specified partition key value will attempt to do a cross partition query by default.