doc: reference: settings: list non-volatile storage backend
doc: reference: settings: list non-volatile storage backend

List the non-volatile storage (NVS) backend in the settings API
reference documentation.

Signed-off-by: Henrik Brix Andersen <a076b11c2cc63ccdc903dbe182e1eecc3cbbc094@brixandersen.dk>

zephyrproject-rtos/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr