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
Checklist
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make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
tests and/or benchmarks are included -
documentation is changed or added -
commit message follows commit guidelines