Update Audio documentation

[aallan]

@tobybroberts can you add a block to the audio documentation which talks about this check,

Run the following command:

$ cat /proc/device-tree/hat/vendor

If it says Raspberry Pi Ltd. then the most you should need (unless your image is ancient/broken) is to install the overlay. If it says IQaudIO Limited www.iqaudio.com then you will need the addition config.txt settings outlined above. If it says words to the effect of "file not found" then the HAT is not being detected, but the same config.txt settings may make it work.

and the associated fixes, see #2891 for full details.