man: add note that emergency.target inherits mount state

man: add note that emergency.target inherits mount state

Based on an internal discussion whether emergency.target should remount disks
ro, or maybe remount them rw, or do nothing. In some cases people want to boot
ro, and always remounting rw would break that. In other cases, remounting disks
ro after they have already been mounted rw is mostly pointless and might even
not be possible. So let's just document that we don't change the state.

Also: any→other, since emergency.service *is* pulled in.

Also: just advertise "emergency" as the way to boot into the target.
We are not going to remove this option, and it's way easier to type than
"systemd.unit=emergency.target".