improve OS classes docs

[nt4f04uNd]

a lot of classes that represent the OS native classes are almost not documented

for example an AVAudioSessionCategory

we have two options how we can handle it:

1. copy all the documenation from the official docs
2. copy the most relevant information (because there's sometimes too much of it) and link as "See also" to the original doc

i personally would prefer the latter

(and of course we can always edit something and add our comments as well)

[ryanheise]

I wouldn't suggest "copying" the docs due to copyright, but maybe it's OK to take Microsoft's approach:

https://docs.microsoft.com/en-us/dotnet/api/avfoundation.avaudiosession?view=xamarin-ios-sdk-12

or at least provide the link to the original docs. I don't consider it the highest priority thing compared to filling in some more of the missing API functionality but yes this is something to eventually do.