Clarify -c. Do not pass it in if you don't know what you're doing

Suggested in #1743

youtube_dl/__init__.py

@@ -358,7 +358,7 @@ def parseOpts(overrideArguments=None):
     filesystem.add_option('-w', '--no-overwrites',
             action='store_true', dest='nooverwrites', help='do not overwrite files', default=False)
     filesystem.add_option('-c', '--continue',
-            action='store_true', dest='continue_dl', help='resume partially downloaded files', default=True)
+            action='store_true', dest='continue_dl', help='force resume of partially downloaded files. By default, youtube-dl will resume downloads if possible.', default=True)
     filesystem.add_option('--no-continue',
             action='store_false', dest='continue_dl',
             help='do not resume partially downloaded files (restart from beginning)')