doc: correct and standardize instructions for disabling TCP servers

doc: correct and standardize instructions for disabling TCP servers

The instructions to set the port to zero to disable a service has not
worked for several years: the effect of doing so is to start the service
on a port randomly selected by the bind(2) system call.

Update the documentation to reflect the new standard way of disabling
network services.

Change-Id: Ic5315a80f9956ea195f63e05d30d604a980bbc8f
Signed-off-by: Peter A. Bigot <pab@pabigot.com>
Reviewed-on: http://openocd.zylin.com/3638
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>