I believe the following CZQL provides all work items in a particular project (change 4tff5jai186bedzidpuk08dvm1 to the external(API) id of your project) with its attached documents:
select name, parent , (select name from Documents) from workitem where project='/project/4tff5jai186bedzidpuk08dvm1'
By the way, you can use describeMetadata with the flag relations as an alternative to describeEntityRelations. Check the REST API reference for more information
Hope this helps,