The link between the workflow workitems and your documents is in table sww_wi2obj (instid = document key, wi_id) the user can be found in table swwuserui using the wi_id from the last table.
The data in sww_wi2obj is for all workitems including completed ones, although you could filter it using the task id or table swwwihead to get the status of the workitem (field wi_stat), this is not required since swwuserwi holds only the active workitem data.