doc: outline when origin is set to unhandledRejection

The `uncaughtException` listener's origin argument was ambiguous
about unhandled rejections. This should clarify when origin is set
to `'unhandledRejection'`.

Fixes: https://github.com/nodejs/node/issues/32907



Signed-off-by: Ruben Bridgewater <ruben@bridgewater.de>

PR-URL: https://github.com/nodejs/node/pull/33530


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>