[3.15] gh-70273: Document default class bindings in tkinter (GH-152389) (GH-152393)

[3.15] gh-70273: Document default class bindings in tkinter (GH-152389) (GH-152393)

Note in the Bindings and events section that every widget inherits Tk
class bindings for its standard behavior, where they are documented, and
how to suppress an unwanted one by returning "break" from a callback.
(cherry picked from commit 4fd69ef97a67fc935e0d4f30662f2a1985ba6be9)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>