Clarify --no-playlist documentation (Closes #4309)

youtube_dl/YoutubeDL.py

@@ -174,6 +174,8 @@ class YoutubeDL(object):
     extract_flat:      Do not resolve URLs, return the immediate result.
                        Pass in 'in_playlist' to only show this behavior for
                        playlist items.
+    no_playlist:       If the URL contains both a playlist and a video ID,
+                       download the video, not the playlist.
 
     The following parameters are not used by YoutubeDL itself, they are used by
     the FileDownloader:

youtube_dl/options.py

@@ -222,7 +222,7 @@ def parseOpts(overrideArguments=None):
     selection.add_option(
         '--no-playlist',
         action='store_true', dest='noplaylist', default=False,
-        help='download only the currently playing video')
+        help='If the URL refers to a video and a playlist, download only the video.')
     selection.add_option(
         '--age-limit',
         metavar='YEARS', dest='age_limit', default=None, type=int,