add small comment to download method in uploads manager

lib/gitlab/import_export/uploads_manager.rb

@@ -97,6 +97,7 @@ module Gitlab
         mkdir_p(File.join(uploads_export_path, secret))
 
         File.open(upload_path, 'w') do |file|
+          # Download (stream) file from the uploader's location
           IO.copy_stream(URI.parse(upload.file.url).open, file)
         end
       end