src: add comment on CallbackScope exception behaviour

PR-URL: https://github.com/nodejs/node/pull/21743


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>