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: https://github.com/nodejs/node/pull/31173#discussion_r362816411

PR-URL: https://github.com/nodejs/node/pull/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>