Answer
You need the record's NoteId first, then query document text through a different path: .../DocumentTextHandling.svc/Reference_DocumentText?$filter=NoteId eq <the_note_id>. Fetch the Customer Order Line's NoteId value first (rather than trying to reference the line's Objkey directly against NoteTexts), then use that NoteId to filter Reference_DocumentText for the actual note/document text content.