n-api: document the look of napi_external values

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

Fixes: https://github.com/nodejs/abi-stable-node/issues/306
PR-URL: https://github.com/nodejs/node/pull/20426


Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>