Add a mention of PYTHONBREAKPOINT to breakpoint() docs (pythonGH-104430)

(cherry picked from commit 1be80ed)

Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>

Doc/library/functions.rst

@@ -168,6 +168,13 @@ 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, 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
 
    .. versionadded:: 3.7