docs: Improve documentation of bootloader usage
docs: Improve documentation of bootloader usage

Describes the necessity to specify the code partition as the chosen one.

Signed-off-by: Benedikt Schmidt <15c2bbfe1d8dcfe05fdbc9b78c35d10937b406a6@embedded-solutions.at>
galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr