gh-70273: Document default class bindings in tkinter (GH-152389)

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.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: serhiy-storchaka

Doc/library/tkinter.rst

@@ -885,6 +885,20 @@ they are denoted in Tk, which can be useful when referring to the Tk man pages.
 | %d | detail              | %D | delta               |
 +----+---------------------+----+---------------------+
 
+The ``add`` parameter above only affects the bindings you make yourself.
+Every widget also inherits *class bindings*
+that implement its standard behavior --
+for example a :class:`Text` widget binds :kbd:`Control-t`
+to transpose two characters.
+These are described in the bindings section of the widget's Tk man page
+(such as :manpage:`text(3tk)` or :manpage:`entry(3tk)`).
+
+Class bindings are processed separately from your own,
+so binding an event yourself does not replace the default; both run.
+To suppress an unwanted default binding,
+bind the event on the widget
+and return the string ``"break"`` from your callback.
+
 
 The index parameter
 ^^^^^^^^^^^^^^^^^^^