illumos gate

We initially documented the basics of mac in #7052. We've documented other unstable MAC properties with #8699 and #8871. To make it much easier to write high-performance device drivers, we should actually document and provide guidance for the implementation of rings and groups. This is not an act of stabilization (every page declares its not stable), but having this will help people more than not.

This covers the use of static groups, which is the main use that we encourage folks to write today.