doc: document change to IncomingMessage.headers enumerability

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

PR-URL: nodejs#42095
Reviewed-By: Matteo Collina <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Robert Nagy <[email protected]>
Reviewed-By: Benjamin Gruenbaum <[email protected]>
Reviewed-By: Harshitha K P <[email protected]>

doc/api/http.md

@@ -2189,7 +2189,7 @@ 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}