doc/releases: Add not on mcumgr support for non-0xff erase flash
doc/releases: Add not on mcumgr support for non-0xff erase flash

The commit adds information, to release notes, on mcumgr now supporting
flash devices that have non-0xff erase value.

Signed-off-by: Dominik Ermel <1a1d45a9cc0c98a37f8d0a0d2dbe3cacc0b2344f@nordicsemi.no>

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