doc: explain native external types

There is a reference to native external types in util.types.isExternal api documentation. Provide a brief explanation to that. Refs: nodejs#31173 (comment) PR-URL: nodejs#31214 Reviewed-By: Rich Trott <[email protected]> Reviewed-By: Colin Ihrig <[email protected]> Reviewed-By: Gireesh Punathil <[email protected]> Reviewed-By: Ruben Bridgewater <[email protected]>
gengjiawen · Jan 8, 2020 · 95871a1 · 95871a1
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diff --git a/doc/api/util.md b/doc/api/util.md
@@ -1392,6 +1392,11 @@ added: v10.0.0
 * Returns: {boolean}
 
 Returns `true` if the value is a native `External` value.
+A native `External` value is a special type of object whose
+data is not stored within the JavaScript managed heap
+and does not conform to standard JavaScript types. Such
+objects are created either by Node.js internals or native
+addons and are wrapped by a JavaScript object.
 
 ### `util.types.isFloat32Array(value)`
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