docs(uffd): fix prefault comment about EEXIST return path

The comment in Prefault claimed faultPage returns EEXIST and "handled
stays false" when a concurrent on-demand fault beats us. faultPage
actually treats EEXIST as handled (returns true, nil), so handled is
true and we still call setState(faulted) + prefetchTracker.Add(...).
The comment now reflects the real semantics.

No behavior change. Addresses PR #2520 review thread (Cursor Bugbot
comment id 3177646485).

Author: ValentaTomas

packages/orchestrator/pkg/sandbox/uffd/userfaultfd/prefault.go

@@ -30,8 +30,10 @@ func (u *Userfaultfd) Prefault(ctx context.Context, offset int64, data []byte) e
 		return nil
 	}
 
-	// Prefault as a read so the page gets WP set. If a concurrent on-demand
-	// write fault beats us, faultPage returns EEXIST and handled stays false.
+	// Prefault as a read so the page gets WP set. faultPage treats EEXIST
+	// as handled (returns true,nil), so a concurrent on-demand fault that
+	// installs the page first is silently absorbed; handled stays false
+	// only when faultPage soft-failed (e.g. UFFDIO_COPY EAGAIN).
 	handled, err := u.faultPage(
 		ctx,
 		addr,