docs(node): Add note to withMonitor that new traces are not created (#15351)

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## DESCRIBE YOUR PR

This will add a note to reduce confusion about `withMonitor` (e.g.
[here](getsentry/sentry-javascript#18029)).

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Author: JPeer264

includes/javascript-crons-job-monitoring.mdx

@@ -8,4 +8,11 @@ Sentry.withMonitor("<monitor-slug>", () => {
 });
 ```
 
+<Alert level="info">
+
+`Sentry.withMonitor()` does not create new a trace. Events and errors that occur within the monitored callback will be associated with the current trace context.
+
+</Alert>
+
+
 If you are using an SDK version prior to `7.76.0`, you can use the `Sentry.captureCheckIn()` API documented below.