Clarify util.types.isExternal

[jdalton]

It doesn't look like there are any unit tests for util.types.isExternal.

I was hoping the docs had a usage example but couldn't find one.

Update:

Found a unit test:

const { JSStream } = process.binding('js_stream');
const external = (new JSStream())._externalStream;

[TimothyGu]

We could put a piece of C++ code for creating a v8::External in as an example.

[cjihrig]

Another alternative would be napi_create_external(), as it's a little more VM neutral than the V8 API.

[apapirovski]

Given the lack of movement here, I'm going to close this out but definitely feel free to reopen if you feel strongly about this. That said, a PR might be the best course of action at this point?

[jdalton]

The docs for isExternal could still be improved. It's still missing a usage example.