Add model_name parameter to OpenAI extra models documentation (#950)

Explain difference between `model_id` and `model_name`. Refs #925.

Author: dguido

docs/openai-models.md

@@ -140,7 +140,7 @@ Let's say OpenAI have just released the `gpt-3.5-turbo-0613` model and you want
   model_name: gpt-3.5-turbo-0613
   aliases: ["0613"]
 ```
-The `model_id` is the identifier that will be recorded in the LLM logs. You can use this to specify the model, or you can optionally include a list of aliases for that model.
+The `model_id` is the identifier that will be recorded in the LLM logs. You can use this to specify the model, or you can optionally include a list of aliases for that model. The `model_name` is the actual model identifier that will be passed to the API, which must match exactly what the API expects.
 
 If the model is a completion model (such as `gpt-3.5-turbo-instruct`) add `completion: true` to the configuration.