Update Doc/library/functions.rst

Co-authored-by: Adam Turner <[email protected]>

Author: nedbat

Doc/library/functions.rst

@@ -168,10 +168,12 @@ are always available.  They are listed here in alphabetical order.
    If :func:`sys.breakpointhook` is not accessible, this function will
    raise :exc:`RuntimeError`.
 
-   By default (if :func:`sys.breakpointhook` has not been replaced), the
-   behavior of :func:`breakpoint` can be changed with the
-   :envvar:`PYTHONBREAKPOINT` environment variable.  See
-   :func:`sys.breakpointhook` for details.
+   By default, the behavior of :func:`breakpoint` can be changed with
+   the :envvar:`PYTHONBREAKPOINT` environment variable.
+   See :func:`sys.breakpointhook` for usage details.
+
+   Note that this is not guaranteed if :func:`sys.breakpointhook`
+   has been replaced.
 
    .. audit-event:: builtins.breakpoint breakpointhook breakpoint