Document the modified queuing of exports

https://redmine.teklia.com/issues/7571

In the 1.6.1 upgrade notes, it was mentioned that exports no longer run without the export queue. This is no longer true.

In CE, the export queue is now useless, since exports never have a source other than default and thus only go to high.

In EE, the export queue is now only for exports on the export database configured with database.export, a setting that is currently documented.

Please document database.export, mentioning that this is an EE-only setting introduced in Arkindex 1.6.0, and mention in the 1.6.2 upgrade notes, a new draft page, the changes to how exports are queued.