Revit API forum on stack overflow would be best. You can also ask in the Autodesk Developer Network if you’re a member.
However the reason such a thing doesn’t exist in the API is because it doesn’t exist in the application.
You can only borrow an element, not a workset. The ‘borrower’ is a note saying that ‘someone owns a thing on this workset’. To get that list of worksets, you have to get the list of things. While the workset element itself can be borrowed in it’s entirety (shown as owner in the table you refer to).