Document `ostree admin finalize-staged` in `ostree admin --help`

[SohamG]

I was digging into how changes to /etc and others are tracked between reboot, which led me to ostree-finalize-staged.service on centos-bootc stream9, which invokes ostree admin finalize-staged. However, I realized that this command isn't documented anywhere, neither man page nor --help.

I think its also a bad look to have a somewhat critical service invoke an undocumented command/feature. I understand that it may be desired that the user never invokes this manually, but that can be indicated in the documentation string.

Thanks