[downloader/external:curl] Clarify why CurlFD should not capture stderr

diff --git a/youtube_dl/downloader/external.py b/youtube_dl/downloader/external.py
index f0c30007f7dd94991eca2e6f15dd5eebcf3e5e3b..cf45562217ac0c27b77f3a241f3881d770d800ce 100644 (file)
--- a/youtube_dl/downloader/external.py
+++ b/youtube_dl/downloader/external.py
@@ -114,6 +114,7 @@ class CurlFD(ExternalFD):
 
         self._debug_cmd(cmd)
 
+        # curl writes the progress to stderr so don't capture it.
         p = subprocess.Popen(cmd)
         p.communicate()
         return p.returncode