document how to exclude files from backups better (team#41627)

 2. add the file(s) to the `/etc/bacula/local-exclude` configuration

    file (lines that start with `#` are comments, one file per line)

That configuration is made in the

The latter is managed by Puppet, use a `file_line` resource to add

entries in there, for example see the `profile::discourse` class which

does something like:

    file_line { "discourse_exclude_logs":

      path => '/etc/bacula/local-exclude',

      line => "/srv/discourse/shared/standalone/logs",

    }

The `.nobackup` file should also be managed by Puppet. Use a

`.nobackup` file when you are deploying a host where you control the

directory, and a `local-exclude` when you do not. In the above

example, Discourse manages the `/srv/discourse/shared/standalone`

directory so we cannot assume a `.nobackup` file will survive upgrades

and reconfiguration by Discourse.

### How include/exclude patterns work

The exclude configuration is made in the

`modules/bacula/templates/bacula-dir.conf.erb` Puppet template,

deployed in `/etc/bacula/bacula-dir.conf` on the director. The files

to be *included* in the backups are basically "any mounted filesystem

that is not a bind mount and one of `ext{2,3,4}`, `xfs` or

deployed in `/etc/bacula/bacula-dir.conf` on the director. 

The files to be *included* in the backups are basically "any mounted

filesystem that is not a bind mount and one of `ext{2,3,4}`, `xfs` or

`jfs`". *That* logic is defined in the

`modules/bacula/files/bacula-backup-dirs` Puppet file, deployed in

`/usr/local/sbin/bacula-backup-dirs` on backup clients.