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

README.md

@@ -93,7 +93,8 @@ which means you can modify it, redistribute it or use it however you like.
                                      COUNT views
     --max-views COUNT                Do not download any videos with more than
                                      COUNT views
-    --no-playlist                    download only the currently playing video
+    --no-playlist                    If the URL refers to a video and a
+                                     playlist, download only the video.
     --age-limit YEARS                download only videos suitable for the given
                                      age
     --download-archive FILE          Download only videos not listed in the

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,