doc: document "http: lazy create IncomingMessage.headers"

Refs: #35281
Refs: #36550

Co-authored-by: raisinten <[email protected]>

PR-URL: #36601
Reviewed-By: Rich Trott <[email protected]>
Reviewed-By: Antoine du Hamel <[email protected]>

Author: ExE-Boss

doc/api/http.md

@@ -1980,6 +1980,12 @@ as an argument to any listeners on the event.
 ### `message.headers`
 <!-- YAML
 added: v0.1.5
+changes:
+  - version: v15.1.0
+    pr-url: https://github.com/nodejs/node/pull/35281
+    description: >-
+      `message.headers` is now lazily computed using an accessor property
+      on the prototype.
 -->
 
 * {Object}