Fixed #22507 -- Clarified nature of the sender argument of signals

This declares a ``pizza_done`` signal that will provide receivers with

``toppings`` and ``size`` arguments.

Remember that you're allowed to change this list of arguments at any time, so getting the API right on the first try isn't necessary.

Remember that you're allowed to change this list of arguments at any time, so

getting the API right on the first try isn't necessary.

Sending signals

---------------

.. method:: Signal.send_robust(sender, **kwargs)

To send a signal, call either :meth:`Signal.send` or :meth:`Signal.send_robust`.

You must provide the ``sender`` argument, and may provide as many other keyword

arguments as you like.

You must provide the ``sender`` argument (which is a class most of the time),

and may provide as many other keyword arguments as you like.

For example, here's how sending our ``pizza_done`` signal might look:

        ...

        def send_pizza(self, toppings, size):

            pizza_done.send(sender=self, toppings=toppings, size=size)

            pizza_done.send(sender=self.__class__, toppings=toppings, size=size)

            ...

Both ``send()`` and ``send_robust()`` return a list of tuple pairs