tcl/interface/ftdi/swd-resistor-hack: clarify and add schematic diagram

Change-Id: I8600ee983de85e4225430ae508a50cd938122d89
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2357
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>