bpo-37039: Make IDLE's Zoom Height adjust to users' screens (GH-13678)

Measure required height by quickly maximizing once per screen.
A search for a better method failed.
diff --git a/Doc/library/idle.rst b/Doc/library/idle.rst
index bd24695..d494c97 100644
--- a/Doc/library/idle.rst
+++ b/Doc/library/idle.rst
@@ -289,7 +289,10 @@
 Zoom/Restore Height
    Toggles the window between normal size and maximum height. The initial size
    defaults to 40 lines by 80 chars unless changed on the General tab of the
-   Configure IDLE dialog.
+   Configure IDLE dialog.  The maximum height for a screen is determined by
+   momentarily maximizing a window the first time one is zoomed on the screen.
+   Changing screen settings may invalidate the saved height.  This toogle has
+   no effect when a window is maximized.
 
 Window menu (Shell and Editor)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^