Clarify behavior of list operations when no results found

[marcboudreau]

This PR adds an explanation that when a LIST operation is issued and there are no results to return, the response is an HTTP 404 response. For example, when no UI headers are configured:

$ curl -v -H "X-Vault-Token: $VAULT_TOKEN" -X LIST "$VAULT_ADDR/v1/sys/config/ui/headers"
*   Trying [::1]:8200...
* Connected to localhost (::1) port 8200
> LIST /v1/sys/config/ui/headers HTTP/1.1
> Host: localhost:8200
> User-Agent: curl/8.4.0
> Accept: */*
> X-Vault-Token: ********
>
< HTTP/1.1 404 Not Found
< Cache-Control: no-store
< Content-Type: application/json
< Strict-Transport-Security: max-age=31536000; includeSubDomains
< Date: Fri, 03 May 2024 17:51:46 GMT
< Content-Length: 14
<
{"errors":[]}

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CI Results:
All Go tests succeeded! ✅

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Build Results:
All builds succeeded! ✅