docs: clarify notes in the config spec
docs: clarify notes in the config spec

ajeddeloh/ignition,dgonyeo/ignition,AlexNPavel/ignition,ajeddeloh/ignition,AlexNPavel/ignition,ajeddeloh/ignition,higebu/ignition,coreos/ignition,coreos/ignition,dgonyeo/ignition,higebu/ignition,higebu/ignition,AlexNPavel/ignition,dgonyeo/ignition,coreos/ignition