docs(java): Add sampling note to OTLP setup (#16555)

## Summary
- Follow-up to #16551
- Documents that the Sentry OTLP propagator inherits the sampling
decision from the incoming `sentry-trace` header, defaulting to sampled
when no flag is present
- Adds a warning that customers may need a custom OTel `Sampler` to
control sampling for deferred traces

## Test plan
- [ ] Verify page renders correctly on the docs site

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Author: adinauer

docs/platforms/java/common/opentelemetry/setup/otlp.mdx

@@ -120,4 +120,14 @@ public class SentryConfig {
 }
 ```
 
-</PlatformSection>
+</PlatformSection>
+
+## Sampling
+
+The Sentry propagator inherits the sampling decision from the incoming `sentry-trace` header. When no sampling flag is present (deferred decision), it defaults to sampled.
+
+<Alert level="warning" title="Deferred Sampling">
+
+If you need to control sampling for deferred traces, you have to implement a custom OpenTelemetry `Sampler`.
+
+</Alert>