doc: release: 3.3: Add note on fixed mcumgr img_mgmt bug
doc: release: 3.3: Add note on fixed mcumgr img_mgmt bug

Adds a note on a fixed mcumgr img_mgmt bug whereby image data could
persist when it should not.

Signed-off-by: Jamie McCrae <7c977670495befe33b67661379bf81cc6b3660ab@nordicsemi.no>

galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr