docs: application: document "make flash" conventions
docs: application: document "make flash" conventions

This is an important part of the workflow, which is missing from the
documentation.

Signed-off-by: Marti Bolivar <9d90ac4c7bf6a305f6bfd81a23c7859bc883380e@linaro.org>

nashif/zephyr,aceofall/zephyr-iotos,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,nashif/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,punitvara/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,ldts/zephyr,kraj/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyriot/zephyr,finikorg/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,mbolivar/zephyr,kraj/zephyr,explora26/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,ldts/zephyr,Vudentz/zephyr,ldts/zephyr,punitvara/zephyr,mbolivar/zephyr,explora26/zephyr,aceofall/zephyr-iotos,ldts/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,mbolivar/zephyr,punitvara/zephyr,explora26/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,nashif/zephyr,nashif/zephyr,galak/zephyr,zephyriot/zephyr,finikorg/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr