doc: application: re-work DT overlay section
doc: application: re-work DT overlay section

Edits and additions for clarity and correctness, based on the current
contents of the file (dts.cmake) which controls the DTS build.

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

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