drivers/flash: protection API clarification

On some targets hardware (or middelware) doesn't allow
implement functionality flash protection API -
so fare it have to be emulated by software on such a target.

This patch changes documentation of this API, so on such a targets
API might implements no-operation.

fixes #15729

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>