[extractor/common] Introduce number fields for chapters and series

youtube_dl/extractor/common.py

@@ -204,21 +204,21 @@ class InfoExtractor(object):
     chapter or section:
 
     chapter:        Name or title of the chapter the video belongs to.
-    chapter_id:     Number or id of the chapter the video belongs to, as an integer
-                    or unicode string.
+    chapter_number: Number of the chapter the video belongs to, as an integer.
+    chapter_id:     Id of the chapter the video belongs to, as a unicode string.
 
     The following fields should only be used when the video is an episode of some
     series or programme:
 
     series:         Title of the series or programme the video episode belongs to.
     season:         Title of the season the video episode belongs to.
-    season_id:      Number or id of the season the video episode belongs to, as an
-                    integer or unicode string.
+    season_number:  Number of the season the video episode belongs to, as an integer.
+    season_id:      Id of the season the video episode belongs to, as a unicode string.
     episode:        Title of the video episode. Unlike mandatory video title field,
                     this field should denote the exact title of the video episode
                     without any kind of decoration.
-    episode_id:     Number or id of the video episode within a season, as an integer
-                    or unicode string.
+    episode_number: Number of the video episode within a season, as an integer.
+    episode_id:     Id of the video episode, as a unicode string.
 
     Unless mentioned otherwise, the fields should be Unicode strings.