[options] Document the syntax for merging formats (closes #3940, closes #4132)

youtube_dl/options.py

@@ -261,7 +261,16 @@ def parseOpts(overrideArguments=None):
     video_format.add_option(
         '-f', '--format',
         action='store', dest='format', metavar='FORMAT', default=None,
-        help='video format code, specify the order of preference using slashes: -f 22/17/18 .  -f mp4 , -f m4a and  -f flv  are also supported. You can also use the special names "best", "bestvideo", "bestaudio", "worst", "worstvideo" and "worstaudio". By default, youtube-dl will pick the best quality. Use commas to download multiple audio formats, such as  -f  136/137/mp4/bestvideo,140/m4a/bestaudio')
+        help='video format code, specify the order of preference using'
+            ' slashes: -f 22/17/18 .  -f mp4 , -f m4a and  -f flv  are also'
+            ' supported. You can also use the special names "best",'
+            ' "bestvideo", "bestaudio", "worst", "worstvideo" and'
+            ' "worstaudio". By default, youtube-dl will pick the best quality.'
+            ' Use commas to download multiple audio formats, such as'
+            ' -f  136/137/mp4/bestvideo,140/m4a/bestaudio.'
+            ' You can merge the video and audio of two formats into a single'
+            ' file using -f <video-format>+<audio-format> (requires ffmpeg or'
+            ' avconv), for example -f bestvideo+bestaudio.')
     video_format.add_option(
         '--all-formats',
         action='store_const', dest='format', const='all',