doc: improve .throws RegExp info

It was not clear why the error name is actually also tested for when
using a regular expression. This is now clarified.

PR-URL: https://github.com/nodejs/node/pull/17585


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>