Update documentation

cmd/image-syncer/README.md

@@ -16,7 +16,7 @@ Contents:
 
 ## Overview
 
-Image-syncer is used to copy container images between two registries.
+Image-syncer is used to copy container images between two registries. Sync the whole index if possible, otherwise sync singular image.
 It copies images **only when they are not present** in the target repository.
 The tool guarantees that **tags are immutable** in the target repository.
 That means that if the image tag is already present in the target registry, it is not overwritten by the new image.

cmd/image-syncer/main.go

@@ -192,7 +192,7 @@ func SyncImages(ctx context.Context, cfg *Config, images *imagesync.SyncDef, aut
 			return err
 		}
 		log.WithField("image", img.Source).Info("Start sync")
-		// sync whole index if possible, otherwise sync singular image
+		// Sync the whole index if possible, otherwise sync singular image.
 		var isIndex bool
 		isIndex, err = isImageIndex(ctx, img.Source)
 		if err != nil {