gh-65802: IDLE - explain SaveAs and extensions (GH-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.
(cherry picked from commit 9890f86ae2001d19e7a18fee5b13aa0dd6069aef)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>