doc: zephyr: Extend info about flashing
doc: zephyr: Extend info about flashing

Add precisions to the information regarding flashing MCUboot and Zephyr
application images in order to make users aware of mass-erase scenarios
that might bite them.

Signed-off-by: Carles Cufi <853081dc8dca4fe6a7e3c984752718b5db36e060@nordicsemi.no>

tamban01/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,utzig/mcuboot,tamban01/mcuboot,ATmobica/mcuboot,utzig/mcuboot,ATmobica/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,utzig/mcuboot,runtimeco/mcuboot,ATmobica/mcuboot,tamban01/mcuboot,runtimeco/mcuboot,utzig/mcuboot,runtimeco/mcuboot,tamban01/mcuboot