doc: Improve ftdi driver section

Fix typo and a sentence that sounds strange since the ft2232 driver
was removed.

Add documentation for the SWD signal usage.

Remove the text mentioning a default VID:PID which doesn't exist.

Change-Id: I27eba571f2f7c46bdb6ae623ab285595018e99d9
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3935


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>