doc: network-management-api: clarify intended event listening usage
doc: network-management-api: clarify intended event listening usage

Clean up the text, add an important warning, and add a self-contained
example rather than referring to the internal test code.

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

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