service/s3/s3manager: Clarify return values for Downloader methods
Clarifies the return value for Downloader's DownloadWithContext and Download methods to be the  size of the object downloaded.
@@ -126,7 +126,8 @@ type maxRetrier interface {
}
// Download downloads an object in S3 and writes the payload into w using
-// concurrent GET requests.
+// concurrent GET requests. The n int64 returned is the size of the object downloaded
+// in bytes.
//
// Additional functional options can be provided to configure the individual
// download. These options are copies of the Downloader instance Download is called from.
@@ -148,7 +149,8 @@ func (d Downloader) Download(w io.WriterAt, input *s3.GetObjectInput, options ..
}
// DownloadWithContext downloads an object in S3 and writes the payload into w
-// using concurrent GET requests.
+// using concurrent GET requests. The n int64 returned is the size of the object downloaded
+// in bytes.
//
// DownloadWithContext is the same as Download with the additional support for
// Context input parameters. The Context must not be nil. A nil Context will
