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)
HarshithaKP · Jan 6, 2020 · 3da691b · 3da691b
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commit 3da691b
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diff --git a/doc/api/util.md b/doc/api/util.md
@@ -1392,6 +1392,9 @@ added: v10.0.0
 * Returns: {boolean}
 
 Returns `true` if the value is a native `External` value.
+`native external` is data that is:
+i) not stored inside JS heap
+ii) not conformant to JS types
 
 ### `util.types.isFloat32Array(value)`
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