doc: release-notes: Add note on mcumgr task status bug fix
doc: release-notes: Add note on mcumgr task status bug fix

Adds a note to the release notes on a bug fixed in mcumgr whereby a
race condition existed that could have gave a false short process
list if a process state changed whilst the list function was running.

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

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