Document changes for async image checks

https://redmine.teklia.com/issues/10673

Requires backend#1909 (closed)

• The new User.can_validate_images user permission should be documented on the user permissions page.
• We do not have any documentation on image server management, so ImageServer.allow_external_validation can be mentioned in the user permissions page as well.
• Both fields and the new RQ queue should be documented in the upgrade notes.