doc: add docs on NodeEventTarget

While researching EventEmitter versus Node.js's EventTarget versus DOM
EventTarget, I found these docs on NodeEventTarget and was confused when
I couldn't find NodeEventTarget itself.

As far as I can tell, NodeEventTarget is used exactly once - by
MessagePort. This makes me wonder if NodeEventTarget should be
deprecated and/or become undocumented - but these docs are a minimal,
hopefully helpful step if that isn't desirable.

Author: joshkel

doc/api/events.md

@@ -2482,6 +2482,9 @@ added: v14.5.0
 The `NodeEventTarget` is a Node.js-specific extension to `EventTarget`
 that emulates a subset of the `EventEmitter` API.
 
+`NodeEventTarget` is not part of the Node.js public API itself but is implemented
+by some Node.js classes.
+
 #### `nodeEventTarget.addListener(type, listener)`
 
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