Apply refurb/ruff rule FURB142

[FURB142]: Replace `for x in y: s.add(docname)` with `s.update(y)`

sphinx/builders/__init__.py

@@ -314,8 +314,7 @@ def build(
 
         doccount = len(updated_docnames)
         logger.info(bold(__('looking for now-outdated files... ')), nonl=True)
-        for docname in self.env.check_dependents(self.app, updated_docnames):
-            updated_docnames.add(docname)
+        updated_docnames.update(self.env.check_dependents(self.app, updated_docnames))
         outdated = len(updated_docnames) - doccount
         if outdated:
             logger.info(__('%d found'), outdated)