Mention i2ctarget and spitarget in busio; make clear how they are implemented

[dhalbert]

https://forums.adafruit.com/viewtopic.php?p=1050272#p1050272 suggests that we document that i2ctarget and spitarget are hardware-based, and also mention them in busio.

[dhalbert]

I think I am not going to mention how it's implemented, since nearly all microcontrollers do provide the hardware.

[dhalbert]

Fixed by #10160.