ref(node): Clarify usage of onFatalError option in node

[kamilogorek]

Fixes getsentry/sentry-javascript#1661 (comment)
Fixes getsentry/sentry-javascript#2796 (comment)

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[MimiDumpling]

Thanks, Kamil! 🌻

[MimiDumpling]

Suggested change

	Be aware that if you are overwriting this setting, you will lose the default implementation, which handles draining queued events before exiting. If you want to see how it works and recreates in your custom code, see the implementation of [`logAndExitProcess`](https://github.com/getsentry/sentry-javascript/blob/master/packages/node/src/handlers.ts) function.
	Be aware that if you are overwriting this setting, you will lose the default implementation, which handles draining queued events before exiting. If you want to examine how it works and then recreate it in your code, see the implementation of the [`logAndExitProcess`](https://github.com/getsentry/sentry-javascript/blob/master/packages/node/src/handlers.ts) function.

[kamilogorek]

Applied both suggestions. Thanks! :)