gh-96197: Define the behavior of breakpoint if sys.breakpointhook is lost (gh-96231)

(cherry picked from commit 09563a76)

Co-authored-by: Dong-hee Na <donghee.na@python.org>

Doc/library/functions.rst

@@ -164,6 +164,8 @@ are always available.  They are listed here in alphabetical order.
    :func:`sys.breakpointhook` can be set to some other function and
    :func:`breakpoint` will automatically call that, allowing you to drop into
    the debugger of choice.
+   If :func:`sys.breakpointhook` is not available to be called, this function will
+   raise :exc:`RuntimeError`.
 
    .. audit-event:: builtins.breakpoint breakpointhook breakpoint