Update the doc for backups and restore.

This change updates the existing doc for the new implementation, including details about it.  The changes reflect the state of things once the new backup and restore implementations have landed.
mongo is state-related, so why is dump separate from root.tar? Do you mean that root.tar contains all files on disk required for running the state server, excluding the mongo database? And dump contains only the mongo database, which requires processing when restoring?