add a mention of PYTHONBREAKPOINT to breakpoint() docs

Author: nedbat

Doc/library/functions.rst

@@ -154,7 +154,7 @@ are always available.  They are listed here in alphabetical order.
    .. versionchanged:: 3.7
       *x* is now a positional-only parameter.
 
-.. function:: breakpoint(*args, **kws)
+.. function:: breakpoint(\*args, \**kws)
 
    This function drops you into the debugger at the call site.  Specifically,
    it calls :func:`sys.breakpointhook`, passing ``args`` and ``kws`` straight
@@ -168,6 +168,11 @@ 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`.  See the docs for :func:`sys.breakpointhook` for
+   details.
+
    .. audit-event:: builtins.breakpoint breakpointhook breakpoint
 
    .. versionadded:: 3.7