include: adc: Fix async ADC API documentation
include: adc: Fix async ADC API documentation

Add information about sequence structure lifespan and fix broken
return value description.

Signed-off-by: Filip Kubicz <065d22bb8773a435670bc9158adcd670a7c08f1c@nordicsemi.no>

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