Currently (arm-image-installer-2.10), the --help lists the following description:
Usage: arm-image-installer <options>
--addconsole - Add system console to extlinux.conf

The --addconsole option with the --target=rpi3 will modify the config.txt to set enable_uart=1 but there is no extlinux.conf to edit as the help indicates
Additionally, the config.txt also has a comment that:
"u-boot will auto detect serial and pass corrent options to kernel if enabled"
So it appears that this is all that is necessary for adding console support to rpi3

If console is enabled but no extlinux.conf is needed, then the help description is confusing.

If the action varies by target, then the description should be generalized.
Maybe
configures system console kernel parameters for the target.

Or of there are limited actions, expand it to list more files that are edited, something like
edits the config.txt or extlinux.conf

A general description can have more details in a /usr/share/doc file or a man page.