vtls: Fixed up include of vtls.h
vtls: Updated comments referencing sslgen.c and ssluse.c
FILE: we don't support paused transfers using this protocol

Make sure that we detect such attempts and return a proper error code
instead of silently handling this in problematic ways.

Updated the documentation to mention this limitation.

Bug: http://curl.haxx.se/bug/view.cgi?id=1286