update picoscroll docs

[helgibbons]

Addresses/fixes #861

I can't get the functions that involve bytearrays to do anything though - are these defunct with PicoGraphics?

[Gadgetoid]

I dare say there's some mix of the old and new happening- it looks like show still exists, but will just flip PicoScroll's internal buffer to the screen, whereas update will pass the PicoGraphics buffer to PicoScroll for display.

This means the bytearray methods probably bypass (and get overwritten by) PicoGraphics style calls. They should probably just be deprecated and removed since there are better, less opaque and more standard ways to show text and images now!