The login commands now expect either `--concierge-mode ImpersonationProxy` or `--concierge-mode TokenCredentialRequestAPI` (the default).
This is partly a style choice, but I also think it helps in case we need to add a third major mode of operation at some point.
I also cleaned up some other minor style items in the help text.
Signed-off-by: Matt Moyer <moyerm@vmware.com>