Add note about 404 result if not enabled

What does this MR do?

The suggested request to https://gitlab.com/api/v4/ml/ai-assist returns the following when code suggestions is enabled for the user and their top-level groups:

{"user_is_allowed":true,"third_party_ai_features_enabled":true}

but when either of those conditions is not met you get the following returned:

{"message":"404 Not Found"}

whereas users may expect to get something like the following:

{"user_is_allowed":false,"third_party_ai_features_enabled":false}

This MR describes the 404 as expected behavior.

Related issues

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