docs(otel): Emphasize that skipOpenTelemetrySetup: true requires manual setup (#15847)

## DESCRIBE YOUR PR


closes getsentry/sentry-javascript#18517

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Co-authored-by: Sarah Mischinger <sarah@codingwriter.com>

Author: s1gr1d

docs/platforms/javascript/common/opentelemetry/custom-setup.mdx

@@ -31,6 +31,8 @@ sidebar_order: 0
 
 Use this guide when you already have a completely custom OpenTelemetry setup or when you intend to add a custom OpenTelemetry setup next to the Sentry SDK.
 
+Setting `skipOpenTelemetrySetup: true` disables the Sentry SDK's automatic OpenTelemetry configuration, **requiring** you to perform the setup manually. For example, to ensure errors are correctly associated with their scope, you must add the `SentryContextManager` to your OpenTelemetry setup. You can find details on the required manual setup further down on this page.
+
 If you are looking to simply add individual OpenTelemetry instrumentation to your Sentry setup, you should read <PlatformLink to="/opentelemetry/using-opentelemetry-apis/#adding-additional-opentelemetry-instrumentation">Adding Additional OpenTelemetry Instrumentation</PlatformLink> instead.
 
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