Clarity in docs about pin ordering

[ATMakersBill]

First, thanks for this library - it will help tremendously.

I'd like to make a small suggestion that might save others some hassle.

The docs say this needs "two GPIO pins" for D+ and D-. Because I knew a bit about PIO, I made sure that the pins were adjacent, but I made the incorrect assumption that the D- pin would be the lower pin (or that order wouldn't matter or would be configurable).

So I got boards made used the QT PY RP2040 board. Because of the way those are mapped, my pins are backward. I don't see a way to fix this in the code (short of rewriting the PIO swapping the pins) so I'll bodge some wires and reorder the boards.

However, it might be nice to be more specific on the requirements. It requires two pins whose GPIO numbers are sequential and where D+ is one less than D-.