Update the doc for backups and restore.
Clarify about the upload-and-restore case.
a -> is.
Explicitly indicate that root.tar does not include mongo-related files.
Clarify the role of --download and --upload.
Drop an unnecessary comment.
Merge pull request #603 from ericsnowcurrently/backups-docs

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.