samples: subsys: mgmt: mcumgr: smp_svr: documentation update
samples: subsys: mgmt: mcumgr: smp_svr: documentation update

This patch aligns documentation to changes in imagetool.py released
in mcuboot 1.2 (latest release).

Also adds note about workaround for possible timeout while erasing
image-slot on some devices (like nRF52840 SoC)

Signed-off-by: Andrzej Puzdrowski <0222c24e1773dd88269ed0e559909ec9bea3604a@nordicsemi.no>

punitvara/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,Vudentz/zephyr,kraj/zephyr,nashif/zephyr,kraj/zephyr,ldts/zephyr,ldts/zephyr,kraj/zephyr,explora26/zephyr,nashif/zephyr,explora26/zephyr,punitvara/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,ldts/zephyr,punitvara/zephyr,punitvara/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,finikorg/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,explora26/zephyr,kraj/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr