CURLOPT_NOBODY.3: clarify this option is for downloads

When enabling CURLOPT_NOBODY, libcurl effectively switches off upload
mode and will do a download (without a body). This is now better
explained in this man page.

Bug: http://curl.haxx.se/mail/lib-2014-08/0236.html
Reported-by: John Coffey
FAQ: 4.20 curl doesn't return error for HTTP non-200 responses
FAQ: mention -w in the 4.20 answer as well