Make migrations more clear in backup documentation

Summary: Not sure this would have avoided the issue, but I remember a couple of other people asking about migrations, so try to make it more clear/obvious that the backup tools are also useful for migrations. Although this is reasonably obvious when you think about it, it's not very obvious when you're trying to do a migration, and maybe making it more explicit will help.

Test Plan: Read new documentation.

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D7992