doc: west: make it clearer how to override the default runner
doc: west: make it clearer how to override the default runner

Though BOARD_FLASH_RUNNER and BOARD_DEBUG_RUNNER are documented in the
debug host tools page, these variables are important enough to deserve
mention in the section dedicated to choosing a runner. Make it so.

Signed-off-by: Martí Bolívar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@nordicsemi.no>

nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr