Improve wording (apply suggestions from code review)

Co-authored-by: dirkf <fieldhouse@gmx.net>

README.md

@@ -128,10 +128,10 @@ Alternatively, refer to the [developer instructions](#developer-instructions) fo
                                          CIDR notation
 
 ## Video Selection:
-    --playlist-start NUMBER              Playlist video to start at (inclusive) (default is
-                                         1)
-    --playlist-end NUMBER                Playlist video to end at (inclusive) (default is
-                                         last)
+    --playlist-start NUMBER              Playlist index of first item to include (default
+                                         is 1)
+    --playlist-end NUMBER                Playlist index of last item to include (default
+                                         is last)
     --playlist-items ITEM_SPEC           Playlist video items to download.
                                          Specify indices of the videos in the
                                          playlist separated by commas like: "--

youtube_dl/options.py

@@ -258,11 +258,11 @@ def parseOpts(overrideArguments=None):
     selection.add_option(
         '--playlist-start',
         dest='playliststart', metavar='NUMBER', default=1, type=int,
-        help='Playlist video to start at (inclusive) (default is %default)')
+        help='Playlist index of first item to include (default is %default)')
     selection.add_option(
         '--playlist-end',
         dest='playlistend', metavar='NUMBER', default=None, type=int,
-        help='Playlist video to end at (inclusive) (default is last)')
+        help='Playlist index of last item to include (default is last)')
     selection.add_option(
         '--playlist-items',
         dest='playlist_items', metavar='ITEM_SPEC', default=None,