add additional preview references

Author: mrbullwinkle

articles/cognitive-services/openai/reference.md

@@ -169,7 +169,7 @@ curl https://YOUR_RESOURCE_NAME.openai.azure.com/openai/deployments/YOUR_DEPLOYM
 
 ## Chat completions
 
-Create completions for chat messages with the ChatGPT and GPT-4 models. Chat completions are currently only available with `api-version=2023-03-15-preview`.
+Create completions for chat messages with the ChatGPT (preview) and GPT-4 (preview) models. Chat completions are currently only available with `api-version=2023-03-15-preview`.
 
 **Create chat completions**