Update app.py (#3962)

Author: elenakrittik

src/textual/app.py

@@ -1959,6 +1959,7 @@ async def push_screen_wait(
         self, screen: Screen[ScreenResultType] | str
     ) -> ScreenResultType | Any:
         """Push a screen and wait for the result (received from [`Screen.dismiss`][textual.screen.Screen.dismiss]).
+        Note that this should only be called when running in a worker.
 
         Args:
             screen: A screen or the name of an installed screen.