Incomplete description for the `model` parameter in the Perform chat completion inference API

Affected pages: https://www.elastic.co/docs/api/doc/elasticsearch/v9/operation/operation-inference-chat-completion-unified

Request body - `model` parameter: The description for the 'model' parameter in the request body of the 9+ version API documentation is missing a detail: "By default, the model ID is set to the value included when creating the inference endpoint." 

