update busio & adafruit_bus_device docs

[2bndy5]

resolves #4927
I also found/fixed some typos while making sure that sphinx's interlinking from busio docs works properly.

[microdev1]

Thanks for the PR @2bndy5.

[tannewt]

FYI @gamblor21 for next time.

[dhalbert]

I was going to make these comments; I'm a little late.

[dhalbert]

I'm suspicious that there's some odd issue, but don't worry about it for the doc purposes. You did what you had to do. I was just helping a user today who seemed to be getting the builtin module in one case and the Python module in another, depending on how the import of adafruit_bus_device was done inside several different libraries. https://forums.adafruit.com/viewtopic.php?f=60&t=180698. That may be related to the linking difficulty you saw, but I'm not sure why.