Update refresh() docstring to document None return type

Co-authored-by: jacalata <2009720+jacalata@users.noreply.github.com>

Author: Copilot

tableauserverclient/server/endpoint/workbooks_endpoint.py

@@ -139,8 +139,8 @@ def refresh(self, workbook_item: Union[WorkbookItem, str], incremental: bool = F
 
         Returns
         -------
-        JobItem
-            The job item.
+        JobItem | None
+            The job item, or None if a refresh job is already queued for this workbook.
         """
         id_ = getattr(workbook_item, "id", workbook_item)
         url = f"{self.baseurl}/{id_}/refresh"