Clarify that download rate is in bytes per second

I found f918ec7ea2 but it is still not clear to anyone who hasn't read Issue #723 whether the limit is in bits or bytes.  This is doubly confusing because 1) ISPs usually advertise speeds in bits per second, and 2) lowercase "k" and "m" are often used in correlation with bits rather than bytes.

youtube_dl/__init__.py

@@ -208,7 +208,7 @@ def parseOpts(overrideArguments=None):
             help='language of the subtitles to download (optional) use IETF language tags like \'en\'')
 
     downloader.add_option('-r', '--rate-limit',
-            dest='ratelimit', metavar='LIMIT', help='maximum download rate (e.g. 50k or 44.6m)')
+            dest='ratelimit', metavar='LIMIT', help='maximum download rate in bytes per second (e.g. 50k or 44.6m)')
     downloader.add_option('-R', '--retries',
             dest='retries', metavar='RETRIES', help='number of retries (default is %default)', default=10)
     downloader.add_option('--buffer-size',