test: refactor comments in test-child-process-spawnsync-maxbuf

* remove comment that isn't relevant/important
* add comment that explains what the test does

PR-URL: https://github.com/nodejs/node/pull/16829


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>