Add missing comment

pkg/modules/pdfengines/multi.go

@@ -101,6 +101,9 @@ func (multi *multiPdfEngines) Split(ctx context.Context, logger *zap.Logger, mod
 	return nil, fmt.Errorf("split PDF with multi PDF engines: %w", err)
 }
 
+// Flatten merges existing annotation appearances with page content, effectively deleting the original annotations.
+// This process can flatten forms as well, as forms share a relationship with annotations.
+// Note that this operation is irreversible.
 func (multi *multiPdfEngines) Flatten(ctx context.Context, logger *zap.Logger, inputPath, outputPath string) error {
 	var err error
 	errChan := make(chan error, 1)