Issue #22820: Explain need for *print* when running file from Idle editor.

Doc/library/idle.rst

@@ -206,7 +206,12 @@ Check Module
 
 Run Module
    Do Check Module (above).  If no error, restart the shell to clean the
-   environment, then execute the module.
+   environment, then execute the module.  Output is displayed in the Shell
+   window.  Note that output requires use of ``print`` or ``write``.
+   When execution is complete, the Shell retains focus and displays a prompt.
+   At this point, one may interactively explore the result of execution.
+   This is similar to executing a file with ``python -i file`` at a command
+   line.
 
 Shell menu (Shell window only)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^