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.
Good point.  My original thoughts were the same as yours.  However, in recent conversations with Horacio (who is doing the restore part) and William, the focus has been only on restoring from a functional environment into a new one, rather than recovering a non-functional environment.  So what I wrote reflects that.  I'll add a note about considerations for the latter point.