[3.10] gh-82006: IDLE doc improvements (GH-94349) (GH-94358)

0. Update text start and stop conditions.
1. Title-case sections but not subsections.
2. Edit Shell Window sections: title, execute, restart.
(cherry picked from commit e6391e08bff775a3c10707fd2cfce6963e6ae429)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
Co-authored-by: Łukasz Langa <lukasz@langa.pl>