doc: document change to IncomingMessage.headers enumerability

The IncomingMessage.headers property was made non-enumerable
in PR #35281.

PR-URL: https://github.com/nodejs/node/pull/42095
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Robert Nagy <ronagy@icloud.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Harshitha K P <harshitha014@gmail.com>

doc/api/http.md

@@ -2197,7 +2197,7 @@ changes:
     pr-url: https://github.com/nodejs/node/pull/35281
     description: >-
       `message.headers` is now lazily computed using an accessor property
-      on the prototype.
+      on the prototype and is no longer enumerable.
 -->
 
 * {Object}