gh-65802: IDLE - explain SaveAs and extensions (#95690)

File name extensions may or may not be shown for the current name
and are added in an OS-dependent manner if not given for the new
name.

Author: terryjreedy

Doc/library/idle.rst

@@ -87,11 +87,14 @@ Save
 
 Save As...
    Save the current window with a Save As dialog.  The file saved becomes the
-   new associated file for the window.
+   new associated file for the window. (If your file namager is set to hide
+   extensions, the current extension will be omitted in the file name box.
+   If the new filename has no '.', '.py' and .'txt' will be added for Python
+   and text files, except that on macOS Aqua,'.py' is added for all files.)
 
 Save Copy As...
    Save the current window to different file without changing the associated
-   file.
+   file.  (See Save As note above about filename extensions.)
 
 Print Window
    Print the current window to the default printer.

Lib/idlelib/NEWS.txt

@@ -4,6 +4,8 @@ Released on 2022-10-03
 =========================
 
 
+gh-65802: Document handling of extensions in Save As dialogs.
+
 gh-95191: Include prompts when saving Shell (interactive input/output).
 
 gh-95511: Fix the Shell context menu copy-with-prompts bug of copying

Misc/NEWS.d/next/IDLE/2022-08-04-20-07-51.gh-issue-65802.xnThWe.rst

@@ -0,0 +1 @@
+Document handling of extensions in Save As dialogs.