Clarify possible reasons for 404 responses

What does this MR do?

It is sometimes causing confusion to customers why the API returns a 404 when a 401 or 403 might seem more natural. Our development guidelines mention the reason for this:

The HTTP status code returned when an authorization check fails should generally be 404 Not Found to avoid revealing information about whether or not the requested resource exists. 403 Forbidden may be appropriate if you need to display a specific message to the user about why they cannot access the resource. If you are displaying a generic message such as “access denied”, consider returning 404 Not Found instead.

This is in line with the RFC as well, but I think it's worth mentioning it a bit more prominently in the user-facing docs.

Related issues

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