Update mark_watched doc

README.md

@@ -92,8 +92,9 @@ Alternatively, refer to the [developer instructions](#developer-instructions) fo
                                          containing directory.
     --flat-playlist                      Do not extract the videos of a
                                          playlist, only list them.
-    --mark-watched                       Mark videos watched
-    --no-mark-watched                    Do not mark videos watched
+    --mark-watched                       Mark videos watched (YouTube only)
+    --no-mark-watched                    Do not mark videos watched (YouTube
+                                         only)
     --no-color                           Do not emit color codes in output
 
 ## Network Options:

youtube_dl/options.py

@@ -188,7 +188,7 @@ def parseOpts(overrideArguments=None):
     general.add_option(
         '--mark-watched',
         action='store_true', dest='mark_watched', default=False,
-        help='Mark videos watched')
+        help='Mark videos watched (if supported for site)')
     general.add_option(
         '--no-mark-watched',
         action='store_false', dest='mark_watched', default=False,