doc: nrf52840_pca10059: re-work programming notes

Mark the three distinct choices for how to program the board as
separate options, and add some clarifications on each. For the MCUboot
route, use smp_svr instead of blinky, to give the user a hint about
how to keep their devices up to date from application land.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>