stream: add comment about undocumented API

PR-URL: #29805
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Minwoo Jung <minwoo@nodesource.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Masashi Hirano <shisama07@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

lib/internal/streams/destroy.js

@@ -1,6 +1,7 @@
 'use strict';
 
-// Undocumented cb() API, needed for core, not for public API
+// Undocumented cb() API, needed for core, not for public API.
+// The cb() will be invoked synchronously if _destroy is synchronous.
 function destroy(err, cb) {
   const readableDestroyed = this._readableState &&
     this._readableState.destroyed;