Ok, thanks. I can see it is partly correct and useful. But the
information about the other function, should not that be shown too?
There is no way to enter just an up-event as argument to C-h k. So,
in the case where the down-event has a binding, there is no way to ask
about the up-event's binding.
Since the up-event will always follow the down-event in practice, it
would be useful for C-h k to mention that the up-event has a binding
too. That would be more convenient and natural than implementing
a separate way to ask for the up-event.