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 <andrzej.puzdrowski@nordicsemi.no>