cli/auth: surface the secret caveat in `auth token` help

The "treat the output as a secret" warning only lived in the Go doc comment, which --help never shows. Now that the command is discoverable, put the caveat in Short (so it appears in the entire auth --help subcommand list) and add a Long + curl Example that carry it in full. Trim the doc comment to point at them instead of duplicating the prose.

Addresses Copilot review comment on #1619.