doc: clarify unhandledRejection events behaviors in process doc

PR-URL: #57654
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Jake Yuesong Li <jake.yuesong@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: dario-piotrowicz

doc/api/process.md

@@ -580,6 +580,10 @@ address such failures, a non-operational
 `resource.loaded`, which would prevent the `'unhandledRejection'` event from
 being emitted.
 
+If an `'unhandledRejection'` event is emitted but not handled it will
+be raised as an uncaught exception. This alongside other behaviors of
+`'unhandledRejection'` events can changed via the [`--unhandled-rejections`][] flag.
+
 ### Event: `'warning'`
 
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