doc: add doc-only deprecation for headers/trailers setters

The headers and trailers for http.IncomingMessage will be read-only in a
future version of Node.js.

Ref: #45571
PR-URL: #45697
Refs: #45571
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: Rafael Gonzaga <rafael.nunu@hotmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: Trott

doc/api/deprecations.md

@@ -3299,6 +3299,22 @@ Type: Documentation-only
 might result in host name spoofing with unexpected input. These URLs will throw
 an error in future versions of Node.js, as the [WHATWG URL API][] does already.
 
+### DEP0171: Setters for `http.IncomingMessage` headers and trailers
+
+<!-- YAML
+changes:
+  - version:
+      - REPLACEME
+    pr-url: https://github.com/nodejs/node/pull/45697
+    description: Documentation-only deprecation.
+-->
+
+Type: Documentation-only
+
+In a future version of Node.js, [`message.headers`][],
+[`message.headersDistinct`][], [`message.trailers`][], and
+[`message.trailersDistinct`][] will be read-only.
+
 [NIST SP 800-38D]: https://nvlpubs.nist.gov/nistpubs/Legacy/SP/nistspecialpublication800-38d.pdf
 [RFC 6066]: https://tools.ietf.org/html/rfc6066#section-3
 [RFC 8247 Section 2.4]: https://www.rfc-editor.org/rfc/rfc8247#section-2.4
@@ -3375,7 +3391,11 @@ an error in future versions of Node.js, as the [WHATWG URL API][] does already.
 [`https.get()`]: https.md#httpsgetoptions-callback
 [`https.request()`]: https.md#httpsrequestoptions-callback
 [`message.connection`]: http.md#messageconnection
+[`message.headersDistinct`]: http.md#messageheadersdistinct
+[`message.headers`]: http.md#messageheaders
 [`message.socket`]: http.md#messagesocket
+[`message.trailersDistinct`]: http.md#messagetrailersdistinct
+[`message.trailers`]: http.md#messagetrailers
 [`module.createRequire()`]: module.md#modulecreaterequirefilename
 [`os.networkInterfaces()`]: os.md#osnetworkinterfaces
 [`os.tmpdir()`]: os.md#ostmpdir