Revert xvid to avi and make docs to be similar to existing external downloader option

youtube_dl/YoutubeDL.py

@@ -262,7 +262,8 @@ class YoutubeDL(object):
     The following options are used by the post processors:
     prefer_ffmpeg:     If True, use ffmpeg instead of avconv if both are available,
                        otherwise prefer avconv.
-    postprocessor_args: Extra parameters for external apps, like avconv.
+    postprocessor_args: A list of additional command-line arguments for the
+                        postprocessor.
     """
 
     params = None