api: counter: Improve documentation of counter_set_alarm().
api: counter: Improve documentation of counter_set_alarm().

Previous description of counter_set_alarm() was insufficient and
could be ambiguously interpreted.

Signed-off-by: Michał Kruszewski <7b290150437d00d973a17f62b9f5454f7732e35d@nordicsemi.no>

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