curl_easy_setopt.3: mention TFTP read callback flaw

The read callback must return the exact requested amount of data when it
is used for doing TFTP uploads. This is due to how it deals with data
internally. This could/should be fixed but for now we document the
existing behavior.

Reported by: Colin Blair
Bug: http://curl.haxx.se/mail/lib-2011-03/0319.html