doc: document common pattern for instanceof checks

PR-URL: https://github.com/nodejs/node/pull/16699
Fixes: https://github.com/nodejs/node/issues/13824


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>