Update 418 status code docs with RFC 9110 reference (#40647)

Author: Shrinivassab

files/en-us/web/http/reference/status/418/index.md

@@ -2,14 +2,18 @@
 title: 418 I'm a teapot
 slug: Web/HTTP/Reference/Status/418
 page-type: http-status-code
-spec-urls: https://www.rfc-editor.org/rfc/rfc2324#section-2.3.2
+spec-urls:
+  - https://www.rfc-editor.org/rfc/rfc2324#section-2.3.2
+  - https://www.rfc-editor.org/rfc/rfc9110#name-418-unused
 sidebar: http
 ---
 
 The HTTP **`418 I'm a teapot`** status response code indicates that the server refuses to brew coffee because it is, permanently, a teapot.
 A combined coffee/tea pot that is temporarily out of coffee should instead return {{HTTPStatus("503")}}.
 This error is a reference to Hyper Text Coffee Pot Control Protocol defined in April Fools' jokes in 1998 and 2014.
 
+While originally defined in [RFC 2324](https://www.rfc-editor.org/rfc/rfc2324) as an April Fools' joke, this status code was formally reserved in [RFC 9110](https://www.rfc-editor.org/rfc/rfc9110) due to its wide deployment as a joke, so it cannot be assigned any non-joke semantics for the foreseeable future.
+
 Some websites use this response for requests they do not wish to handle, such as automated queries.
 
 ## Status