n-api: document the look of napi_external values

Describe what JavaScript values of type `napi_external` look like in
JavaScript.

Fixes: nodejs/abi-stable-node#306
PR-URL: #20426
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>

Author: Gabriel Schulhof

doc/api/n-api.md

@@ -1051,6 +1051,9 @@ the ECMAScript Language Specification.
 In addition to types in that section, `napi_valuetype` can also represent
 `Function`s and `Object`s with external data.
 
+A JavaScript value of type `napi_external` appears in JavaScript as a plain
+object such that no properties can be set on it, and no prototype.
+
 #### napi_typedarray_type
 ```C
 typedef enum {