[ai-assistant] Make it clear that the API key must be private (#19244)

Author: oliviertassinari

docs/data/data-grid/ai-assistant/ai-assistant.md

@@ -66,7 +66,7 @@ The Data Grid provides all the necessary elements for integration with MUI's ser
 1. Contact [[email protected]](mailto:[email protected]) to get an API key for our processing service.
 
    :::warning
-   Avoid exposing the API key to the client by using a proxy server that receives prompt processing requests, adds the `x-api-key` header, and passes the request on to MUI's service.
+   Do not expose the API key to the public. Instead, keep it private, use a proxy server that receives prompt processing requests, adds the `x-api-key` header, and passes the request on to MUI's service.
 
    This is an example of a [Fastify proxy](https://www.npmjs.com/package/@fastify/http-proxy) for the prompt requests.