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: nodejs#32907

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

PR-URL: nodejs#33530
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/process.md

@@ -233,8 +233,9 @@ changes:
 
 * `err` {Error} The uncaught exception.
 * `origin` {string} Indicates if the exception originates from an unhandled
-  rejection or from synchronous errors. Can either be `'uncaughtException'` or
-  `'unhandledRejection'`.
+  rejection or from an synchronous error. Can either be `'uncaughtException'` or
+  `'unhandledRejection'`. The latter is only used in conjunction with the
+  [`--unhandled-rejections`][] flag set to `strict` and an unhandled rejection.
 
 The `'uncaughtException'` event is emitted when an uncaught JavaScript
 exception bubbles all the way back to the event loop. By default, Node.js
@@ -2604,6 +2605,7 @@ cases:
 [`'exit'`]: #process_event_exit
 [`'message'`]: child_process.html#child_process_event_message
 [`'uncaughtException'`]: #process_event_uncaughtexception
+[`--unhandled-rejections`]: cli.html#cli_unhandled_rejections_mode
 [`Buffer`]: buffer.html
 [`ChildProcess.disconnect()`]: child_process.html#child_process_subprocess_disconnect
 [`ChildProcess.send()`]: child_process.html#child_process_subprocess_send_message_sendhandle_options_callback