System.IO. (file stuff)

I was looking for some documentation on writing data out to files, and I found 1 thread which used calls to System.IO.xxxx

I can’t see any mention of this in the unity documentation though. How do I find out the specifics of this API?

System.xxx is always Mono / .NET, not unity at all

As such you find documentation on it at microsofts MSDN and at go-mono
in both cases you will want to look for the C# documentation normally