Touch Display installation needs better instructions

[lurch]

The photo at https://www.raspberrypi.com/documentation/accessories/display.html#physical-installation doesn't seem very clear as to exactly which pins are being connected to which. Especially given that the pin-labels on the Touch Display's PCB aren't readable once a Raspberry Pi has been screwed onto the standoffs!

We should also add some wording about "if the touchscreen printed circuit board is powered directly", similar to that shown in https://datasheets.raspberrypi.com/display/7-inch-display-product-brief.pdf