doc/device_mgmt/mcumgr/img: Correct note on "upgrade" parameter

The note previously mentioned that, when "upgrade" flag is present,
uploaded image version can not be lower than version of a running
application; this has been corrected as uploaded image should
have version higher than running, as "upgrade" only flag makes
no sense when same version is uploaded.
Additionally note that Zephyr does not support this feature has
been removed, as MCUmgr library within Zephyr does actually
perform the version check when "upgrade" flag is set.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>