[BUG]: Update command doesn't update list of cores for currently running resen

[asreimer]

After running the update command, the list of available cores within the currently running resen session is not refreshed.

This is because the update_core_list method doesn't call the __get_valid_cores method after successfully downloading the new cores.json file.

This means that users need to restart the resen shell before creating a new bucket.

Also, the documentation is completely missing any information about the update command.

I'll make a quick PR to fix these issues and we can do a minor release.