Documented specific case when UseSentryTracing must be called manually

[jamescrosswell]

DESCRIBE YOUR PR

In the specific use case decribed here when people are using WebApplicationBuilder but need/want to control when the EndpointRoutingMiddleware is added, they need to manually call UseSentryTracing.

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PRE-MERGE CHECKLIST

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• Checked Vercel preview for correctness, including links
• PR was reviewed and approved by any necessary SMEs (subject matter experts)
• PR was reviewed and approved by a member of the Sentry docs team

[vercel]

@jamescrosswell is attempting to deploy a commit to the Sentry Team on Vercel.

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[jamescrosswell]

@bitsandfoxes any idea what the Secret Scan is and why this PR might be failing that check?