doc

Make the 'offset' parameter optional, if omitted simply start at the
beginning of the flash bank.

Additionally, check if the argument is out of bounds of the flash bank.

Change-Id: Id1959eee5c395666c35f26342c3c50134dd564e5
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3858


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>