Joypad interrupt behavior is described incorrectly

Pan Docs currently [claims](https://gbdev.io/pandocs/Interrupt_Sources.html#int-60--joypad-interrupt) the following:

>The Joypad interrupt [is requested](https://gbdev.io/pandocs/Interrupts.html#ff0f--if-interrupt-flag) when any of [P1](https://gbdev.io/pandocs/Joypad_Input.html#ff00--p1joyp-joypad) 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.