tools: make doc tool a bit more readable

PR-URL: https://github.com/nodejs/node/pull/17125


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