Documentation: Updates for Omeka S 4.0

[jimsafley]

Per #27, we should recommend that users turn off File Sideload's automatic deletion when performing batch create/update operations. Otherwise, should an error occur during the operation, the module may delete the files before the corresponding items are created.

[allanaaa]

This isn't a 4.0 change, right? I can push this immediately?

[jimsafley]

Yeah, sorry, this is not specific to 4.0.