Joypad interrupt behavior is described incorrectly

[calc84maniac]

Pan Docs currently claims the following:

The Joypad interrupt is requested when any of P1 bits 0-3 change from High to Low.

However, based on multiple sources over the years in the gbdev Discord, I've heard that the actual behavior is that the interrupt is requested when the state of P1 bits 0-3 changes from all High to at least one Low. I'm not sure if there's a public document stating this, so updating Pan Docs is probably the best course of action here.