tools: add docs for prefer-util-format-errors rule

I had a little trouble understanding what the rule was trying to say, so
am documenting what would pass/fail.

PR-URL: https://github.com/nodejs/node/pull/17376


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>