[YoutubeDL] Document youtube_include_dash_manifest

youtube_dl/YoutubeDL.py

@@ -304,6 +304,12 @@ class YoutubeDL(object):
                        otherwise prefer avconv.
     postprocessor_args: A list of additional command-line arguments for the
                         postprocessor.
+                        
+    The following options are used by the Youtube extractor:
+    youtube_include_dash_manifest: If True (default), DASH manifests and related
+                        data will be downloaded and processed by extractor.
+                        You can reduce network I/O by disabling it if you don't
+                        care about DASH.
     """
 
     _NUMERIC_FIELDS = set((