doc: application: improve Kconfig setting documentation
doc: application: improve Kconfig setting documentation

Now that the above sections more clearly define the behavior of
CONF_FILE, the section on writing a .conf can be cleaned up and made
into a simple primer on syntax.

Signed-off-by: Marti Bolivar <fd5da480b758a78208111e6ae5c6806e1f887137@opensourcefoundries.com>

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