docs(core): Document SDK backfilling processor marker

Clarify that SDK-owned backfilling processors must also use the Sentry
processor marker so callback error handling can identify their ownership.

Refs #6081
Co-Authored-By: Claude <noreply@anthropic.com>

Author: adinauer

sentry/src/main/java/io/sentry/BackfillingEventProcessor.java

@@ -4,5 +4,8 @@
  * Marker interface for event processors that process events that have to be backfilled, i.e.
  * currently stored in-memory data (like Scope or SentryOptions) is irrelevant, because the event
  * happened in the past.
+ *
+ * <p>SDK-owned implementations must also implement {@link
+ * io.sentry.internal.eventprocessor.SentryEventProcessor}.
  */
 public interface BackfillingEventProcessor extends EventProcessor {}