RESUME_FROM: clarify what ftp uploads do

The numerical value passed to CURLOPT_RESUME_FROM for FTP uploads is
interpreted and used as position where to resume the _reading_ of the
local file and it will "blindly" append that data on the remote
file. This was certainly not clear in the docs previously.

Reported by: catalin
Bug: http://curl.haxx.se/bug/view.cgi?id=3048174