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: #31173 (comment)

PR-URL: #31214
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/util.md

@@ -1391,6 +1391,11 @@ 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)`
 <!-- YAML