Update documentation to include os-specific functionality

[ifox12]

Hey there,

I'm experimenting with a http server, and I wanted to fork() the process when accepting a connection, since I didn't find anything in the package doc I looked into the Odin src and found os/os_linux.odin. There I found fork(), tried it and it works.

So, if a function/API is available for Odin users, shouldn't it be documented as such? Or - and my guess is this would be preferable - shouldn't there be an os-agnostic fork() function in core?

By the way, the same seems to apply to other linux system functions.