doc: clarify exceptions during uncaughtException

PR-URL: https://github.com/nodejs/node/pull/5180


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Benjamin Gruenbaum <inglor@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>