Updates to several documents regarding backup_mode to make it more visible, as well as a backup_mode documentation type fix.

updated the faq to add a note about backup_mode, fixed a typo in the backup mode docs, and updated one of the file.managed examples with a backup line, as well as an additional note directing the user to the backup mode documentation in a highly visible area. This resolves https://github.com/saltstack/salt/issues/9058