Clarify doc strings

aptly/interfaces.go

@@ -130,7 +130,7 @@ type Downloader interface {
 	DownloadWithChecksum(ctx context.Context, url string, destination string, expected *utils.ChecksumInfo, ignoreMismatch bool, maxTries int) error
 	// GetProgress returns Progress object
 	GetProgress() Progress
-	// GetLength of given url
+	// GetLength returns size by heading object with url
 	GetLength(ctx context.Context, url string) (int64, error)
 }