Fix inaccurate OpenAI-related documentation

[andrewigk]

See here: https://docs.spring.io/spring-ai/reference/api/chat/openai-chat.html#openai-compatible-servers

There are several mentions of the official OpenAI API ignoring unknown parameters such as 'extra_body' when provided in the request body.

This is not true, and should be addressed. Entire commits of code have been based on this assumption, but has no one truly verified for themselves to discover this is not the case?

Any and all unknown/unsupported parameters passed to the OpenAI API will result in a 400 error code and an error message like this:

"error": {
    "message": "Unknown parameter: 'extra_body'.",
    "type": "invalid_request_error",
    "param": "extra_body",
    "code": "unknown_parameter"
  }